T-Mobile allows their pre-paid customers to roam, while AT&T's
pre-paid customers only get use of AT&T's own network.

Here's AT&T's coverage map:

http://www.wireless.att.com/coverageviewer/?WT.svl=title

The pre-paid customers don't get coverage in any area marked as
"Partner".

Here's T-Mobile's pre-paid coverage map:

http://www.t-mobile.com/coverage/Prepaid.aspx

T-Mobile offers roaming service on 850MHz in some areas.

If you already have an unlocked GSM phone that works on the US
frequencies, it's sometimes still cheaper to buy a complete pre-paid
phone package at a discount store like Target or WalMart and simply
take the SIM card out and put it in your own phone.

==snip==

Yes -- there are about 100 companies in the US that offer this kind of
service. The lion's share of these companies are on Sprint (CDMA) but
about 20 are GSM. If you're coming to the US for a month you might have
a local contact and I'd suggest you rely on them.

You indicate that you want to buy a pre-paid SIM for use with your
unlocked GSM phone. The T-Mobile stores want $50 for this. You can buy
the same package online for $15 (Lewis Wireless) -- best to get your
local contact to do this. (This could take a few days for shipping,
etc.) The Cingular (now AT&T) prepaid phone and SIM is $19 at Radio
Shack and $18.43 at WalMart (both stores are everywhere in the US.) $15
will buy 60 minutes of airtime from Cingular and keep the phone active
for 30 days.

T-Mobile is a few dollars more up front ... but the phones are nicer and
airtime is cheaper and the phones stay active longer.

Because a North American dual band phone is not very useful to you in
Thailand ... I suggest you give it away as a bread and butter gift in
the US ... if this happens at the end of your trip, you could give them
the SIM too.

There are sales, too. $15 and some airtime for T-Mobile ; $10 and some
airtime at Cingular (Virgin offers a free phone with as little as $10 in
airtime..but they're CDMA in the US -- on the Sprint network.)

Regards, Gordon