You can't prepay for mobile internet usage with any of the carriers, as far as I know. Alltel (at least they did last season) would allow you to subscribe to a month-to-month plan provided you either owned your own connection equipment (PC card or USB) or bought it and paid for it in full from them. You could then terminate your contract after any amount of time, whether or be 1 month or 1 year, without a penalty. I bought a card off of eBay and then used this last year.
I had pretty decent coverage in the TX Panhandle with it. I'm not sure what the status is with their merger with Verizon, but that could only improve coverage. Last year, Alltel also had a roaming agreement with Sprint, so you could use their towers, too.
Again, this is all about what was available last year. I have no clue about this season. The only way to know for sure is to call each of the cell companies and ask them. Also, keep in mind that some of the companies have caps on their usage. AT&T has a 5GB cap, I think.
