Of course since it seems that there is so little in low cost data services here in the US, it may not work out quite as wonderfully as the $15/month the poster has mentioned, but it is still a nifty concept to consider.
I guess it is time now to watch for whatever best deals may come around for AT&T based services with a very cheap data rate, and consider trying this idea out.
Any related & helpful thoughts on this topic will be greatly appreciated.
Dennis Bournique's website Prepaidcompare.net may come in handy. You can browse by desired network (eg; AT&T) and options (eg; unlimited slow data).
Black Wireless (AT&T MVNO) "Data Plus" plan for $15/month (or less with "multi-month" savings) includes: 200 minutes / 200 texts / 500 MB data at high speed, then unlimited data throttled at 128 kbps.