ATT's SIM card will also work nationwide.  Last I checked (four months ago
or so), the Cingular SIM was regional.  ATT's required monthly recharge.  If
you go for the $40 recharge rather than $20 recharge, you get free nights
and weekends. ATT's service permits off network roaming at $0.25 a minute in
cities it doesn't cover and in Canada.

If your friend is going to be calling the UK a great deal, might I suggest
getting a prepaid international long distance card.  Calling the UK is 2.5
CENTS a minute from onesuite.com. It is $1.59 a minute from ATT.

If your friend opts for ATT, they have a web deal going right now which
includes the phone and the SIM for $29.  This cheaper than the SIM only
deal.  Just buy it and put the SIM in his own phone.  Check first to make
sure that the phone doesn't support the new(ish) GSM 850mhz band.  If you
travel to the US regularly, you get much better reception and coverage with
a quad band phone or with a phone that handles the other US GSM frequency.
Many of the rural carriers (e.g. Dobson and Centennial) have licenses on
this frequency and are rolling out GSM.  Further, in place like NYC, this
frequency is more open (e.;g. less users at the moment).  Lastly, like
900mhz GSM, 850gm had better building penetration than the higher
frequencies.

Here is the link to the prepaid plan on the T-Mobile website:

www.t-mobile.com/promos/prepaid/default.asp

Boy, they burried the link to their prepaid service.  Whether T-Mobile is cheaper than ATT depends on the usage pattern, but I agree that for the casual users, T-Mobile is cheaper.  For the heavy user, I think ATT works out cheaper becaue of the free nights and weekends, but the chance that you jump back on the plane to the UK with a fair amount left on the US SIM seems pretty high.