I have yet to try them out, but Clickatell seems like a nice, developer friendly SMS gateway.
They offer a trail account where you get a few messages for free (although they override the message content with a default text), so I couldn’t resist and tried it out.
It’s really a matter of minutes if not seconds to get started:
Date: Tue, 23 Jun 2009 11:59:22 GMT
Server: Apache/2.2.3 (Red Hat)
I have not yet compared their prices with other gateway providers, but I don’t have a real need for it now.