I use t-mobile also and as long as you're willing to purchase $100 refills, it's 10 cents per minute, but if you want to purchase $30 monthly, it's much more expensive. They have basically have 2 plans. The one I'm on is 10 cents a minute for calls, and 5 cents for texts. The other one, I think has a daily fee of $1, but you get free nights/weekends, which may be good for a college student, if they can manage that block of minutes properly. You can buy refills at a discount at callingmart.com, and I've seen fatwallet coupons for them in the past for additional discounts. I think the $100 initial refill I bought cost me around $92 on callingmart.com. But you really do have to buy them in the $100 increments with t-mobile 2 go to get the cost per minute down to 10 cents per minute. The lower denomination refills are much more expensive per minute.
My friend looked into this for a visiting student they have, and he ended up finding an AT&T reseller called airvoice wireless, but it hasn't worked out for him yet. He bought a SIM off ebay, and they haven't been able to activate it in the 808 area code. They are supposedly looking into it. He wasn't in a hurry because I gave him a net10 phone with a bunch of airtime left on it that I bought when I lost my GSM Razr on my California trip in July. Anyway, they are worth a look, especially if you already have an unlocked phone, so you can try it at minimal cost to see if it works for you.
The t-mobile 2 go plan works great for me because I'm a very light cell phone user, so basically 1000 minutes will last me an entire year. So after the cost of the phone, I'm basically on an $8 - $9 a month cell phone plan. I bought an unlocked quad-band GSM Razr off ebay for around $100, rather than use one of their phones, but you can do it either way. They have the usual crappy $30 locked phone packages at Walmart or on their web site or in their stores. If you have your own unlocked GSM phone, you can order a SIM on their web-site for around $5 - $7, I think, which is what I did.
If you want to do more research, I got started with prepaid after reading some threads on it in FT's technology forum.
-David