I stick by my initial reply to the OP. I recommended Orange which is what I used 3 weeks ago in my US Blackberry and for a measly 5 quid, I got a month of data usage which was perfectly acceptable and reasonably cheap phone calls. I think from memory I topped it up with 20 quid at the time and came back to the US still with 10 quid remaining.
By my putting an Orange Sim in, I did not divulge my new temporary number to anybody in the US - cos I didn't want to be bugged so there were no roaming charges.
If anybody wanted me - they could email me.
Worked for me.