I hate to say it but I cannot figure it out. Looking in System Preferences > Keyboard and Mouse > Keyboard Shortcuts shows some commands that don't seem to do much at all.
Command + Option + ` says it "Moves focus to the drawer", but the Finder doesn't have a "drawer" in the regular sense, rather it has the drawer's replacement, a sidebar. You'd think that command would move the focus there.
Pressing Tab just selects the next icon for some stupid reason.
^F5 moves the focus to the Search box which would make you think you could then press Tab to tab to the next area. In iTunes, Tab does this. Selects the Search box, followed by the Playlist followed by the Sidebar. You'd think Apple would have been smart and caught this. I am thinking it's a bug and oversight. I mean Mail acts the same way. Search Box > Message List > Sidebar. Finder.. well let's hope Snow Leopard's new Finder fixes it. Seems like a silly bug. And a GUI consistancy overlook. BTW, iCal seems to act completely differenty in this case. Tab moves from Search to the Sidebar, but then it makes the selected item editable, then tabs into the calendar from one event to another. Such a strange behavior.