What kind of reward cards are you talking about? Like this one?
http://www.rewardscards.com/ATT-Universal-Card.asp
I hope you're not talking about credit cards. I would not feel right about keeping that kind of info on my phone unless it was password-locked. Even then, just... no. I don't see what advantage that would give you other than just having those numbers handy; you couldn't use it like a normal credit card because there's no magnetic strip on the iPhone (in addition to several other missing features... I know, it's not like any other phone has it anyway).