I have to echo the goldfish sentiment... it really is very cruel. You'll find a million horror stories on pretty much any wedding messageboard about using goldfish as favours, centerpieces, etc. They may be small, but they still are a living creature.

That said... I'm having a fall themed wedding, and our favours are going to be wooden fall "leaves" with our names and the date etched on them. We'll either put a magnet on the back, or drill a small hole in them to be used as ornaments. FH's mom is making them, and they're so cool... they look really realistic!