TMobile used to offer (maybe they still do?) a data-only plan that was/is meant for remote sensing applications (alarms, etc). It wasn't something that was listed on their site (ok - it was there, but you had to really, really dig for it to find out about it).

IIRC, it was also free, but limited to 200 MB a month.

You basically got a SIM card you could stuff into your embedded sensor app and away you go.

That might be a large enough data cap for plain-text messaging, but not much else.