The O2 international SIM deal is probably your best option Lizzie - I was going to suggest Lycamobile, but their US rates have gone to 5p a minute now.
Hope this information assists...
BR / Steve
[size=2]Steve Gold
PO Box 1014, Sheffield S10 5YG, UK
Home mobie: Telefonica O2
Other UK mobiles: 3, Vodafone, Virgin
Foreign SIMs: Toggle (multi); Germany (Fonic); Poland (Orange PL);
Skype: stevewgold