Google announced in a Google Groups thread that it has added arrow key navigation to Gmail.
Google technical program manager Richard John Wu writes:
Now, you can use the keyboard arrow keys to navigate your labels (the navigation panel on the left that includes the Inbox, Sent Mail, and Drafts links), and also to navigate the conversation threadlist (the message list panel on the right) in Gmail.
When you navigate using these keys, the browser will focus on selected items, enabling screenreaders to read out what's selected.
Previously, using the arrow keys would have caused your browser to scroll up or down the webpage.
In other semi-Gmail related news, Google just added Google Calendar to the Gmail Search Field Trial, and a Gmail Attachments feature has been spotted in Google Drive.