Share 'Best Phone-Modem-Prepiad SIM Card Combination for USA Usage'

After much pain with FrontlineSMS and several phone/modem/SIM combinations, I'm wondering what the best setup is for users in the USA. I'd like to run FLSMS on a Windows XP computer using a pre-paid SIM card. I'd prefer T-Mobile as I already have a line and credit to use, but I'm not tied to them. What is a requirement is a USB modem or phone w/ USB cable that a) really works on US cellular carr…