Delving into GSM/GPRS

I bought a GSM/GPRS board today. Seems like interfacing to Arduino will be pretty simple via serial port.

However, the means for obtaining a SIM for it is not clear nor is the most cost effective route to go. I intend to issue periodic, infrequent SMS or small email messages (this is an alarm warning notification application so there better not be very many!).

Anyone in the US been through the search for cheap, low data volume airtime sources?

Seems like T-Mobile might get me into a roughly $20/year payment pattern with their offerings, based on some reading I've done. But that sounds really cheap and so I'm not that confident the source was accurate. In that scenario, you buy a by-the-minute plan, fork over $100 to get into their 'gold' level of service and then switch to a pay as you go. The $100 doesn't expire for a whole year and you need to do a refresh of $10 a couple times a year. Yeah, the 'entry fee' is a bit salty.

If that is true, then great. But if not, what have the rest of you found cost effective?