For prepaid, the United States unfortunately has few options.
So, to make things the easiest for those accustomed to Europe/Asia systems: if you're used to SIM cards and 3G on an unlocked phone you're bringing yourself, the easiest to deal with is AT&T. They play on the right frequencies, and use WCDMA standard most everyone else does. It won't be cheap!
Verizon and Sprint are a NO GO: they use the US-centric version of voice and 3G, based on CDMA: no SIM cards, no GSM
SimpleMobile, as mentioned before, is somewhat flawed. It seems like a good deal if you're a GSM user for voice, text, 2G. However, they use the T-Mobile 3G system on the 1700 Mhz band, which is unusual. (In short: no iPhone will work on this using 3G, nor will most international 3G phones like Samsung Galaxy S II, etc.).
There may be other MVNO or 2nd tier operators for cheaper, but I wouldn't bother unless you really know what you're doing. For most of the world, prepaid is widely used by normal folks and travelers. In the US, it's targeted towards low-income, risky phone users, so there's not a lot of premium choices here.
WARNING: US cell phone store workers are typically quite incompetent. That you can bring a phone from overseas and pop in an AT&T SIM chip and work is still quite mysterious and bizarre to them. I'm not kidding. Expect to know a lot more about phones than they do. Most Americans have never travelled abroad and have no idea how the rest of the world lives.
