I mentioned this earlier, but as a "Backup" phone I use Virgin Mobile.
They have a $35 a month plan (I happen to be lucky enough to be grandfathered at $25) that is unlimited Android data, with 300 anytime minutes.
If you are in 3g or WiFi, you can use the Skype app to make free calls and not chew your 300 minutes. Works great.
I have to stick with T-Mo because of my travels, but if I was US only, in areas that Sprint (the provider for Virgin) covered, I'd save the bucks and go with Virgin Mobile full time.