This might be old news for some of you, but it wasn't to me, so here it is...
I just discovered this today (after having used Entourage since 2001, and Outlook Express since 1999): you can cycle through the panes in Entourage's main window with the Tab key, and then use the arrow keys to navigate between items (i.e. folders in the left pane, and mails in the upper right/middle pane, depending on where you have the preview pane). Shift-Tab reverses the direction of your focus' movement. This allows me to browse my mail with the keyboard which is great because I tend to be a keyboard control type of person anyway.
Of course, there's always Command-1 for mail view, Command-2 for your adress book, and Command-3 for the calendar, plus the OS X standard Command-~ for cycling through windows.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060327225056393