I've been casually looking around for data-only plans with no (or really low, at least) monthly fees and pay-as-you-go per MB data... I'm not coming up with much, but roamline [ https://www.roamline.com/roamline-sim ] might do the trick. No monthly feed, twenty eurocents per MB.

Truphone [ http://www.truphone.com/us/consumer/sim/ ] is another one I've seen pop up. No monthly fee, $0.09 per MB in the US.

Does anybody know of anything else? This seems like a market that typically buys access for thousands of devices at a time, pushing just a few MB per month. There's not much out there for the hobbyist looking for just a SIM or two.