I asked a similar question recently since I am new to the GSM world. From what I've found, you can use any GSM phone for the ATT network, any GSM phone for the T-Mobile network if you can get it unlocked, or any already unlocked GSM phone. The one caveat is the phone has to use at least the 850/1900 MHz bands.
I also wanted a simple phone I could take snowboarding, hiking, etc. I did some searching and found the Motorola v197. It's an unlocked Quad band 850/900/1800/1900 phone so you can take it anywhere in the world. It has miniUSB connectivity so you can charge it off your computer. It has bluetooth for hands-free use if that's important. It has a monochrome outer display and a color inner display. It's pretty compact aside from its thickness and feels pretty rugged. It has a large battery for the size/features of the phone so it has a long standby time. It also seems that you can do some rudimentary hacking to it from what I've read. I think I paid somewhere around $50 for it.
I've used it a couple times when I've gone on bike rides where I didn't want to carry my iPhone and it's been fine.
