It was in the news how Verizon decided to sell off a part of altell back in 2009:
AT&T ready to pick up some Alltel assets from Verizon « Boy Genius Report
But its not all of Alltel..aka some of it will remain so you prob could get your TP2 working on Verizon.
As for sprint..your phone wont work due to ESN restriction but if service down there is good it may be worth it..let me explain..
Sprint has something called EDRP which costs $110 and $15 each line..
That would end up $155 a month..comes with 1600 total minutes, unlimited data, unlimited mobile to mobile any carrier and unlimited texts.
There are a few phones you can get free or on the cheap (mostly from 3rd parties).. example: Android based Samsung Intercept for $5 in wirefly with new contract...there are also 3 new Android phones coming..as for windows mobile..its pretty much dead..you can prob ebay a TP for fairly cheap these days though or get a TP2 for $100