I've always been annoyed that the only way to mark all as read for a mailbox was with a contextual menu in the sidebar - lets change that.

I use Butler as my quicklaunch keyboard macro tool - but you could use your favorite one to run the following script while reading mail.

#!applescript

tell application "Mail"

activate

get message viewer 1's selected mailboxes

repeat with thisBox in result

set read status of (messages of thisBox whose read status is false) to true

end repeat

end tell

I find this almost critical for getting through mailing list traffic.