A while ago, I threatened to post this script, and then it slipped my mind, mostly because I switched to Gyazmail. At any rate, one of the most frustrating things about Mail.app to me was having to get out the mouse to switch between the inboxes of my various email accounts, especially once I got Mail Act-On working so nicely. The applescript was harder to figure out than it perhaps should have been, so here it is in the hopes that it will help someone.

tell application "Mail"
        set the selected mailboxes of the front message viewer to {mailbox "INBOX" of account "Gmail"}
        delay 0.1
        set no_selected to get count selection
end tell
 
if (no_selected is equal to 0) then
        tell application "System Events"
                set selected of row -1 of table 1 of scroll area 1 of splitter group 2 of window 1 of application process "Mail" to true
        end tell
end if

Paste the script into Script Editor.app, change Gmail to whatever you have your account named, and save it to ~/Library/Scripts. You’ll need to use something like Quicksilver or FastScripts to set up a keyboard shortcut, and then away you go.