I don't know if this has been submitted before, but I just noticed that you can attach your vCard from Address Book to your signature in Mail.
Open Mail -> Preferences -> Signatures and select Add Signature, then open Address Book and drag your vCard to the open signature window. Add whatever other text you want and hit OK to save it. This is a great feature that I wish I'd noticed before.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030121062514304