technically speaking, you can tether with your V710. It would be slow though. The V710 uses 1xRTT, which is about the same speed as the iPhone running on AT&Ts EDGE network (though i give a slight edge to EDGE). I wouldn't bother with it though. not worth the effort.

Sprint really is the cheapest with their SERO plan. $30 for 500 min, $50 for 1250 min and $100 for 2500 minutes. Unlimited data on all. But their voice service is widely regarded as weaker than Verizon's. As for phones... Palm Centro is cheapest @ $99 and the hack to tether for free is available, should you choose that route. I would suggest the Mogul though. More expensive @ $299 but it has Bluetooth 2.0 which helps tethering speeds and the device will be upgradeable to EvDO Rev. A in early 2008 if not sooner.

If you don't use a hack, everyone will charge $60 to tether. It's not cheap.