As far as I'm concerned - if I'm in the showdown, part of what I've paid for to get to that point is to see the other guy's hand. Bad form or no, I'll get that info every time.

Technically - anyone at a table can ask to see an unshown hand. You will be tossed from a casino for asking too many time to see a hand IF you were not in the showdown. You will never be penalized for asking to see an opponents hand if you were actually in the showdown with him. Your opponent may not like having to show - but if he doesn't want to show he should duck out before getting a chance to win the pot...