Hi. I am going to be in the USA for the next couple of months, in San Francisco, and I wanted to get a local, pre-paid cell phone plan for my basic connectivity needs. I have a GSM phone, and I just need to be able to make and receive very few calls, and use 3G (and use the web, tethering my phone to my N800!)

Does anybody have a suggestion of what plan I could get?

U dont have much choice.

If you want to use your own phone, most likely you need to use AT&T (3G) prepaid sim card.

Since very unlikely you have 3G phone for t-mobile (unless your phone is pentaband - Nokia N8).

My suggestion is to get a cheap AT&T phone (prepaid) and just slot in the simcard to your phone.

Added: Assuming your phone is pentaband:
http://www.t-mobile.com/shop/plans/prepaid-plans.aspx

The N900 supports T-Mobile USA's 3G frequencies. On the other hand, it does only EDGE on AT&T.

I've been told that AT&T prepaid SIMs lock to the phone in which they are activated, unlocking automatically only after six months have passed. I have no idea if this is true, but if you choose to go AT&T, it might be better to activate the SIM in the phone you really want to use just in case.

I use AT&T prepaid with my N900 as there's no T-Mobile coverage where I live. If you want more information on AT&T's data costs or the like, just ask.

Well, I'll be definitely using GSM phones, both the one I'll take and the one I plan to buy there eventually (yes, I am very hopeful the Harmattan phone will become available while I'm there! ). I don't care much about the connection speed, EDGE is perfect, but the locking thing would certainly blow!...

I will check out the frequencies my phone supports. So AT&T and T-Mo are really the only two with GSM networks, right?

Pretty much, yes.

There are still small, regional GSM carriers in a few places, but they're few and far between, and last I looked, none of them appeared to offer much in the way of data plans. I don't know that San Francisco is home to one of those in any case.

Beyond that, we have a few MVNOs (Simple Mobile being one) that resell either T-Mobile or AT&T's network. They're less likely to have roaming agreements in place to allow you to roam onto the other network though, but that may not be a concern in San Francisco. The coverage there should be decent with either AT&T or T-Mobile.

I used T-mobile while in SF last year in my N900 without troubles. The only issue is that I was unwilling to stump up the quite considerable pre-payment for data, which sort of took away the fun. Perhaps things have changed now?