First off I should explain my perspective: I am a T-Mobile customer. I don't buy data service for my N900 as I live and work too far from urban areas to get data coverage. When I want data service while traveling, what works best for me is to bring my laptop to a McDonald's or a Coffee shop that has free wifi, buy an overpriced beverage, and camp at a table for a while.
There are currently primarily two separate voice hardware specifications used in USA, CDMA2000 and GSM. I don't know of any phones that will talk to both CDMA and GSM.
The USA nationwide carriers that use CDMA are Verizon & Sprint. US Cellular, Alaska Communications Systems, and others use CDMA in some geographically restricted markets, and smaller carriers that sell access to the larger carriers networks also exist. If it's contractually allowed, CDMA phones can roam for voice calls on another carrier's CDMA network.
The major carriers in USA for GSM are ATT & T-Mobile (& GCI in Alaska). ATT won't sell you a SIM card unless you have made an agreement to have a contract with them which will last for a year or more. ATT seems not to want you to buy service from them on any phone other that one they sell, and doesn't want you to have any service on a data capable phone unless they sell you data service on it.
T-Mobile phones can roam on ATTs nework. I have the unlimited talk & text plan from T-Mobile, and it has paid for calls I have made from everywhere I've been that there's GSM signal, except Unalaska Island in the Aleutians. If I'm near the border with Canada, T-Mobile will also remove any charges incurred if my phone accidentally gets picked up by a Canadian tower. I suppose roaming could work the other way, but a comparison of ATT's and T-Mobile's coverage maps will show that the question is somewhat moot.
The carriers have coverage maps on their web sites. No one sells prepaid cellular data service in USA. One of the cheapest Data plans is from Virgin Mobile who sells pre-paid voice on Sprint's network. A great resource for prepaid cellular purchasing are the general purpose electronics stores Radio Shack & Best Buy. They're not the least expensive, but the convenience is often worth the expense. General purpose merchants (eg WalMart, etc) also sell prepaid phones inexpensively, but the selection is much more limited.
If you plan to roam, make sure that you purchase a plan of the "nationwide no roaming" type as if you make calls while roaming without such an agreement, you've given the operator of the network on which you roam permission to financially rape you, and they likely will.
If you plan to buy additional time for a prepaid wireless phone, I recommend buying it online from someone like www.wirelessrefill.com I've found that the online prepaid airtime merchants have better rates. When you purchase time, they email you a code. You enter the code on your phone, hit send, and when the phone is done communicating with the network, it has the extra time. It's also worth noting that prepaid airtime expires if unused. The amount of time it takes for the unused time to expire positively correlates to the amount airtime purchased in one chunk.