This is a great suggestion from lifehacker.

I have loads of hotel, airline and other membership cards. In fact, I don’t have a wallet big enough for them all so they sit in a drawer at home. I used to have a list of the numbers somewhere, but it quickly became out of date.

One thing I do have with me 95% of the time is my iPhone. And now, as I’ve scanned all my cards and put them in a gallery in Aperture and sync’ed with my iPhone, I have all my cards with me when I need them!

The only two issues I see are, 1, don’t just credit cards, drivers licence, etc. as your phone may be lost/stolen, and 2, it’s difficult to swipe the mag stripe! :)

Here’s the original article.