That sounds like an interesting idea, tmchiba. If someone liked my costume enough to give me one of those, I'd be puzzled at first (what? no camera?!
), but ultimately I'd feel kinda honoured. Like, I'd remember you more for the card than if you just took a picture and didn't say anything.
On the other hand, if EVERYONE started doing that, my bags would be filled with cards and I'd get sick of carrying them around. But if that's the only downside to your idea that I can think of, you should go ahead and try it.
Good luck with your idea!