Just click the "View" link at the bottom of a Gmail message and the viewer will take it from there. If you decide you want to edit the file, clicking "Edit online" will open it in Google Docs, or you can download it to your desktop from there.Links:
http://gmailblog.blogspot.com/2010/06/view-doc-attachments-right-in-your.html
http://buzz.blogger.com/2010/06/new-share-buttons.html