In Mail 3.0, in your current mailbox view, you can jump to any message by typing the first letters of the from name or the subject line. This also works for RSS feeds, notes, and to do items.
[robg adds: Previously in Mail, the only way to navigate the inbox view via the keyboard was via the up and down arrow keys.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071029151926418