Simple HTTP post to send an SMS with an SMS API
nexmo.com
nexmo.com
There's other complications that they don't seem to handle that well (Text encoding! Bulk sending!). But they may have some luck - their website is easy on the eye, their APIs look sane, and the pricing isn't so bad.
As far as text encoding - it's a hideous world, and I'm glad to forget the details. But you should probably give a link on your documentation to the GSM03.38 character set, and specify what happens if you include something that isn't on it, for a start. And maybe provide optional support for non-GSM03.38 encodings.
(I might as well ask this here :^)