1. The +1 button will influence search rankings. Here is the exact quote from Google's David Byttow, from when the feature was first announced: "We'll also start to look at +1's as one of the many signals we use to determine a page's relevance and ranking, including social signals from other services. For +1′s, as with any new ranking signal, we'll be starting carefully and learning how those signals affect search quality over time."
2. When a user searches, while signed in, their search result snippets may be annotated with the names of their connections who have "+1′d" the page. When none of the user's connections have +1′d a page, the snippet may display the aggregate number of +1′s the page has received.
3. Google says publishers could see "more, and better qualified traffic coming from Google" as potential visitors see recommendations from friends and contacts beneath their search results
4. Google calls the +1 button "shorthand for 'this is pretty cool' or 'you should check this out'.
5. One a user clicks the button, a link to the content appears under the +1′s tab on the user's Google Profile.
6. Google suggests clicking the button when you "like, agree with, or want to recommend" something to others.
7. The +1 Button is not the same as Google Buzz, though there are similarities. They both appear on your Google Profile under different tabs, but +1′s don't allow for comments (at least yet. I would not be surprised to see Buzz's functionality get rolled into +1 eventually).
8. +1′s are public by default. Google may show them to any signed-in user who has a social connection to one. Users can choose not to have them displayed publicly on their Google Profile, however.
9. There are different sizes and styles of the button that you can use on your site.
10. The button is even more customizable if you want to get more technical. The API documentation can be found here: http://code.google.com/apis/+1button/
11. When a user clicks on the +1 button it applies to the URL of the page they're on.
12. Still, multiple buttons can be placed on a single page that all +1 different URLs (refer to the above documentation).
13. While Google suggests you use the button where you think they'll be most effective in terms of placement around your content, the company recommends above the fold, near the title of the page, and close to sharing links. Google also says it can be effective if you put it at the end of an article as well as the beginning.
14. By placing the <script> tag at the bottom of the document, just before the body close tag, Google says you may improve loading speed of the page (which is another factor Google takes into account in terms of ranking).
15. If you try to +1 a private URL, it won't work, according to Google.
16. You have to be logged into a Google account for the button to work.
17. While everyone can see aggregate annotations, signed in users can also see personalized annotations from people in their Gmail/Google Talk Chat list, My Contacts group in Google Contacts, and people they're following in Google Reader and Google Buzz.
18. Google points to these canonicalization strategies to ensure the +1s "apply as often as possible to the pages appearing in Google search results." http://www.google.com/support/webmasters/bin/answer.py?answer=139066
19. The button is supported in 44 languages (though the annotations only appear in the English language Google.com search results for the time being).
20. The button will be seen in the Android Market, Blogger blogs, Product Search, and YouTube, in addition to any other sites that add them.
21. A lot of sites have already replaced the Google Buzz button on content pages with the +1 button
22. If you have a Blogger blog, you can add the button by going to Design > Page Elements on the dashboard, finding the "Blog posts" area, clicking edit, and selecting the "Show Share Buttons" options, where you should find the +1 button as an option.
23. The +1 Button will be available on YouTube watch pages under the "share" feature. Consider how valuable YouTube can already be to SEO, and take then take into consideration the search implications of the +1 button.
24. If you're signed into your Google account, Google will show you +1 annotations from your Google contacts on YouTube search results.
25. Google says adding +1 buttons to your pages can help your ads stand out on Google. "By giving your visitors more chances to +1 your pages, your search ads and organic results might appear with +1 annotations more often. This could lead to more–and better qualified–traffic to your site," the company says.
26. The +1 button will appear next to the headline on search ads. Personalized annotations will appear beneath the Display URL.
27. Publishers can get updates about the button by joining this group.
28. Google may crawl or re-crawl pages with the button, and store the page title and other content, in response to a +1 button impression or click.
29. Google has strict policies for publishers that it says it will use (along with the Google ToS) to govern use of the +1 button. Here are these policies in their entirety:
Publishers may not sell or transmit to others any data about a user related to the user's use of the +1 Button. For the avoidance of doubt, this prohibition includes, but is not limited to, any use of pixels, cookies, or other methods to recognize users' clicks on the +1 Button, the data of which is then disclosed, sold, or otherwise shared with other parties.
Publishers may not attempt to discover the identity of a +1 Button user unless the user consents to share his or her identity with the Publisher via a Google-approved authorization procedure. This prohibition includes identifying users by correlating +1 Button reporting data from Google with Publisher data.
Publishers may not alter or obfuscate the +1 Button, and Publishers may not associate the +1 Button with advertising content, such as putting the +1 Button on or adjacent to an ad, unless authorized to do so by Google.
Publishers may not direct users to click the +1 Button for purposes of misleading users. Publishers should not promote prizes, monies, or monetary equivalents in exchange for +1 Button clicks. For the avoidance of doubt, Publishers can direct users to the +1 Button to enable content and functionality for users and their social connections. When Publishers direct users to the +1 Button, the +1 action must be related to the Publishers' content and the content or functionality must be available for both the visitor and their social connections.
Google may analyze Publishers' use of the +1 Button, including to ensure Publishers' compliance with these policies and to facilitate Google's development of the +1 Button. By using the +1 Button, Publishers give Google permission to utilize an automated software program (often called a "web crawler") to retrieve and analyze websites associated with the +1 Button.
30. The button is not available on mobile search results yet, though users may still be able to see the buttons on your pages.
31. According to Search Engine Land, while they may still be a while away, Google will launch analytics for the button, to show webmasters info on geography, demographics, content, and search impact. Apparently Google is working with launch partners to make sure reporting is accurate before they offer it on a wider scale.
If you want the code for the button to add to your site, you can get it here.
There are more discussions (including issues people are having with the +1 button in Google's Webmaster Central Help Forum.
http://www.webpronews.com/google-1-button-3-2011-06 (source from webpronews.com)
http://gmailblog.blogspot.com/2011/06/our-plans-to-support-modern-browsers.html (from gmail blog)