RepublicWireless.com just went live with all the details. Here's the catch (they're quite transparent about it):

The phone monitors how much data you're sending over Wifi vs cellular, and if you're using a disproportionate amount of cell data compared to the rest of the community, they can boot you.

"The best way to know how you're doing is by checking out your Cellular Usage Index (CUI). If it's too high, we'll let you know and give you tips to bring it down. You have plenty of time. But meanwhile, you still pay a flat fee of $19/month no matter what."