people sell SIMs on eBay that are discounted because they are closer to expiration.
Most of the T-Mobile prepaid SIMS on eBay without minutes are sold unactivated, often for $1-2, and have full time once you activate them. The preactivated ones with lots of time on them are usually only recently activated, and tend to have over 10 weeks remaining before the next top-up. The sellers are often people who work at T-mobile retail shops, who get these SIMs as a perquisite of the job from T-Mobile. I bought one this way last year, it had 400 minutes for $25 and didn't expire for 80 days. (However, I just looked, and there don't seem to be any like this listed now.) The biggest downside to buying a preactivated card is that the area code might be something wierd.
Tmobile is currently selling a Nokia 2610 phone for $29 (or a Motorola V195 for $39), and including $25 in air time. I don't know if they honor this in stores. The Motorola is a good phone (my son has one). - DvF
And every Wal-Mart, Target, and Best Buy in the country has a good selection of prepaid phones from T-Mobile and others. My new school is in a place where my current (Sprint) phone is roaming ($$$+$$$+$$$+), so until I permanently get there and settle in with a new phone, I got an Alltel cheapo for about $25 with $10 in included airtime and 60 bonus minutes for online activation. Then I bought a $20 card (They chard $.15/minute) so I should be good to go.
Before you get here, check the coverage maps for all of them--T-mobile, Alltel, US Cellular, Sprint, Virgin Mobile (which uses the Sprint network), Cingular/AT&T, and TracPhone. None of them would be a problem in any major metropolitan area (I take that back--I live in a major metro area now and when we moved here our Verizon phones wouldn't work), but if you're going someplace out of the way, one may be decidedly better than another.