For prepaid, the US basically stinks -- few options, expensive, US-only standards, kill you on data/3G.

* 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. As Alan mentioned above, they play on the right frequencies, and use WCDMA standard most everyone else does. It won't be cheap, so we gotta make sure our Wikimania folks give very good Wifi to use. :)

* 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.

Anyone who wants to put this on the Wikimania wiki, feel free.

-Andrew