My wife and I are loving Ting so far. I have been spreading the word to family and friends, and most seem interested in Ting's model. However, several people were uncomfortable/unhappy with the absolute lack of roaming data. I realize that providing roaming data is very expensive, but wonder if some people would be willing to pay substantially more for it under certain conditions. If your arrangement with Sprint doesn't preclude data roaming, perhaps you could add a separate billing category for it, and charge whatever is required to recoup what it costs Ting when people roam. You could even make it a separately-enabled feature on the account management page. Then if someone ABSOLUTELY needed it, they would have it, but would know ahead of time that 50MB would cost them $10 or whatever.
Just a thought...