I'm always a bit surprised this doesn't some up more often in these discussions. I used to use little serial GSM/GPRS modems anytime I needed to handle SMS, and that was while working for a voip company with lots of infrastructure in place for that kind of thing (the modems always won based on simplicity, speed, and consistency).

At the small scale spending $100 or so to have an unlimited test system seems comparable, and for large scale I'd much rather be buying a few unlimited SMS plans than paying per message. If the wireless provider supports SMS over GPRS you can do something like 30/min so you may never need more than one modem and sim. I wonder if it is just the barrier to entry, since you have to know that such a thing is possible and hunt down hardware/software to fit your needs rather than just buying in to one of these companies' pitches.