It doesn't really pay for itself but here are some good uses I've found (after being vehemently anti-iPhone):

1) For the geographically challenged - Navigation. Walking around NYC for 2.5 weeks unsupervised, the iPhone's Google maps function + free subway map were invaluable tools. Kept me from looking like a tourist rube, got me home safely, and I didn't have to nag friends for directions everywhere.

2) For the chronically lazy - email. I can check my email without getting out of bed. That may or may not be a plus.

3) For school - tools. The HP 12C calculator was not only a pain to find, it was pricing out at $120. The HP12C app cost $14.99.

4) For the Heroes + coupon fan - Yowza! You can search for local coupons on this free App.

5) For the hungry - Urbanspoon. This free app, while not perfect, can give you a quick and easy overview of local restaurants, organized by price ranges, cuisine, and neighborhood. And it's organized like a slot machine, so that's fun, too.

None of these pay for the iPhone, but they're useful enough to sort of justify the increased cost. A little. Oh, I know the Android has a program where you can scan UPCs and run a price comparison online. A friend has used it to get pricematched electronics. I haven't checked but there may very well be an app for that on the iPhone, too.

If I ever figure out a way for it to generate revenue, I'll let you know!

You could save money shopping with coupons, price comparison, and shopping lists.

I wrote this 16 Ways Your iPhone Can Save You Money post on my old blog as tongue-in-cheek, but if you're already spending money for the things I list, then in theory the iPhone really could help you save money on them.