I'm also interested to hear what options are around. I'm primarily interested in sending texts, to phones in USA, Canada, and Mexico, including prepaid phones. The US in particular seems hard for this, with certain carriers only and odd rules (like no prepaid on T-mobile from one of the suppliers). I can see the exceptions leading to lots of support questions!
Also nice to have, but not essential, would be an incoming number in the US (or Canada) that accepts SMS programatically (note: Google Voice is very poor at accepting SMS, especially international, not an option). The US short codes seem to be very expensive, bit of a luxury for my application.