A user from MacOSX.com was having trouble adding a vCard to his signature in Mail 2.0 in Tiger. He was trying to drag and drop the vCard, but Mail wasn't accepting the drag.
So as I investigated, I found that it was definitely a bug in Mail 2.0. However, the workaround is quite easy (and this works for any attachment of any type):
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050515155524583