T-Mobile has some good prepaid GSM packages. If you need a new phone (i.e. if your foreign phone doesn't do the 1900 band) I've seen new Nokia 3390s with US SIM and a small amount of starter airtime for $49 to $59.

Your local T-Mobile store should be able to hook you up with a new prepaid SIM for $20 plus airtime, around 40 cents per minute for USA calls. Top-up cards are readily available at gas stations, convenience stores, newsstands and the like.

T-Mo's postpaid plans are much cheaper if you make a lot of calls (my plan is 1000 daytime domestic weekday minutes + unlimited domestic night & weekend minutes for $39.99/month plus tax) but they all require a one-year contract.