Careful...
The only network that an iPhone can work on in the US is AT&T in 3G. For 2G you can go with T-Mobile or AT&T (T-Mo USA use a frequency unsupported by the iPhone for 3g.)
However...
Caveat 1) with a legally unlocked O2 iPhone, every time you put a non-O2 simcard in, it locks the phone out and forces you to connect it to iTunes. When you connect to iTunes, it identifies the sim card then gets the carrier settings and permissions (APN etc) - they claim so you don't have to fiddle. (Down side - if your iPhone runs out of battery you have to connect it to iTunes)
Caveat 2) AT&T only allows data use with a contract SIM card on iPhones, and they enforce this by the above mechanism disabling data use on the phone. So if she's going to get a PAYG sim, and hope to use data - she's going to be dissapointed.
However - if it's jailbroken (or never locked by a carrier), this doesn't apply.
That's said in the big american cities, google maps is supremely useful in getting round, as it links up with public transit systems - so well worth jailbreaking IMHO
Discovered this myself on a recent trip
