When I signed up for Sprint (who I will pimp here - excellent customer service, coverage, and plans), I asked about deposits. I signed up under a parent (not quite 18... few more days...), so I didn't have to put one down, but I was told if I did, it was usually about $100, and it was held for 11 months. I guess if you cancelled or reneged on payments, they kept it. Otherwise, they gave it back.
But yeah. Sprint rocks. Also, when you go shopping, ask about business discounts (assuming you have a job). Sprint gives discounts ranging from 5-25% just for working at certain places. For instance, since I work at Sears, I get a 25% discount. That's a pretty nice bonus. No idea if other providers do this, but it's worth asking.
Definitely shop around, though. I hit every single provider, barring U.S. Cellular (sucks), SunCom (heard horror stories), and Boost Mobile (need I say more?). My mom has Cingular, and I hate them. Their customer service is unhelpful, their coverage isn't all that great once you get out of metro, and they didn't have especially dazzling plans. Verizon rocks, don't get me wrong, but you'll pay for it. I would have gone with them, but Sprint had the discount going for them. Plus, the local rep I bought the plan from is quite possibly the nicest and most helpful sales guy I've ever had. Answered all sorts of questions for a good month before I actually bought the phone, gave me tips on discounts and such, and even charged my phone for me one day while I was at work, (both in the mall) and it was dying. Oh yeah, he rocks
Regarding Elec's bit about texting, yeah. I bought the 500 plan ($8/month) for that purpose. I have idiot friends who like texting. Turns out they apparently have yet to get my number, which is a good thing. But I've started texting like mad anyway, since I have 'em. Also of note, if you think you'll be doing any sort of web usage, and I mean any, buy a plan. Sprint charges $0.02/KB, which you'd think wouldn't be much for a quick Google here and there. Wrong. I racked up $16 my first month. Immediately bought a data plan, which is a whopping $10/month for unlimited usage. First month free, to boot.