In case anybody reading this thread is looking for just data service
(for lots of data, like 20GByte or even unlimited), I recommend having a look at millenicom.com. I have been using them for a year now and they hooked me up to the Verizon network with 20GByte data for $60 a month and I can stop it whenever I don't need it. Reactivate for $20.
I see they now sell unlimited data for $70 a month which is on the Sprint network IIRC.
For voice we just use Skype over this data service