11:19 pm
I am in the same predicament (live in Canada, but go to the states every couple of months). I use Virgin Mobile. It's 20c/minute, but the minimum you need to top up is $60 per year, provided you register a credit card with them (Otherwise it's $80 per year)
Seems like T-mobile gets you a better deal per minute, BUT virgin has the "sugar mama" service. If you log onto their website for 3 minutes every day and watch some flash ads, it gives you extra minutes that don't expire! (up to 65 minutes every month) I spend some time doing that while I'm in Canada, and I never have to worry about running out of airtime when I'm in the States.
Additionally, you can get a basic LG flip-phone from them for free, while T-mobile makes you pay for yours.
I highly recommend.