One curious thing about a data-only phone is that, via Android and Google Voice, it might well still be a perfectly functional phone, at least so long as you're in 3G coverage land. That could make for a cost-effective solution.
Woah, gotta stop you there. Sadly, Google Voice still uses your cellular voice network on your end. I believe it's VOIP from Google to the recipient, but it's still the regular voice network from Google to you.
So yeah, Google voice still uses your minutes. But like I said, SMS is completely free to and from your phone.