I have several email accounts in Mail (I'm using Tiger), along with multiple signatures per account. In the new message window, if you pick the current account from the account selection pop-up menu, Mail will randomly re-select an email sig for you (if you have your preferences set to randomly pick one).
This is handy if you don't like the one that was randomly selected...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050611012006961