I use Tru (formerly Truphone) which covers UK, US and Oz, I assume through the sorts of deals that these guys have pulled off.

It's a fabulous idea but badly executed (despite the fact that they have raised oodles of VC money) with poor call quality and slow (2G) data speeds. Still, so much easier than buying a SIM in the US. Also, has the wonderful property that you can call someone who is physically in the US on their UK mobile number for free for both parties - for which incoming call (for those of you who don't know how backwards the US cell market is in comparison with regulated Europe) a US cell customer would normally have to pay!

That said, if you mainly travel to the US and don't want to deal with T-Mobile or Virgin or whoever, I heartily recommend them.