Originally posted by Maciek Nowakowski, Associate Product Manager (from gmail blog)
When I check my email, I often rely on the message snippets to figure out which messages to open first. Sometimes, though, I want to see more than snippets, which is why I'm happy to announce that you can now preview messages in your inbox using a new feature in Gmail Labs called Preview Pane. It's probably a very familiar layout to those of you who have used Gmail on a tablet device. We also think it's going to work especially well if you have a larger resolution screen.
For those of you who have more vertical space you can also move the preview pane below your message list. You can enable this using the dropdown arrow next to the toggle button:
By default there is a 3-second delay in marking a conversation as read after previewing it. If that doesn't feel natural to you, you can change the timing in the General tab of settings: