one word: nightmare
think of all the different platforms, email applications, etc. add that to the fact that some people think HTML-formatted email is the work of the devil him/herself. i wouldn't do it, even if a client demanded it from me.
i would, however, email them a link where they could pick the card up. you keep people happy and it's far easier for you, from a developers point of view.
derbs should be able to chime in from experience on this one, if i remember correctly?