Yes, of course you can charge an iPhone with a USB power source, but you can't run an iPhone on USB power. If you use your iPhone the battery level will fall even if it's charging. I haven't tried this yet, but maybe you could just squeak by used if you turned off 3G and WiFi.
This isn't just a curiosity; it means that if your iPhone is in the "dead zone" of low battery level, you can't use it again until it charges out of the critical low level.
Presumably the iPhone needs more power to operate than USB 2 can provide. Alas, if it could still work with firewire that would be enough, and I suppose USB 3 could work.
I wonder if the Pre, which is said to be more efficient, can both charge and operate off of USB power.
Update 6/24/09: I read recently (Dan's Data) that newer laptops can provide significantly greater current and voltage than the standard USB spec. They do that to support power hungry devices. I think my iPhone charges more quickly from my new corporate Dell laptop than it does from my (non-Apple) USB charger. The next time I'm running low, I'm going to try charging it from the Dell laptop during use, and see I can stay out of the dead zone.