I'm testing a different solution. Alltel has been absorbed by the Borg Verizon, so I'll lose Alltel's unlmited connectivity. The cheap solution, here, is Cricket who won't permit tethering and only has one USB modem the tablet won't power. So, I bought one of these:
http://www.zatznotfunny.com/2008-02/...o-router-ever/
to try out. It's about $200 online.
Plug modem into EVDO router, plug router into car battery and sit in rear window of car. Connect tablet to your own mobile wifi within about 120' of car. Bingo! Problem solved, so far. $40 Cricket gives me about 400-700Kbps and, so far, hasn't complained about my 10GB/mo+ consumption, even though they say they're going to throttle it over 5GB/mo. Hasn't happened so far.
I can't imagine the USB port on a tablet EVER going to power a sellphone modem.