Week one of Google SoC 2007 is already over and I'm off to a great start of my SMS Framework project. I've begun developing the gateway side of the API. I've designed it so that additional gateways can be created and install like standard modules. Gatway modules implement hook_smsframework_info(), which returns an array containing information about the gateway(s) the module enables. The information includes the gateway's name, callbacks (for validation, sending, ect.), and a form for configuration.
I ran into trouble while planning for the Drupal side. I discovered that the subscriptions module is not an API (I don't know why I'd assumed that). So I will need to develpoemt a module that does much of the same things as subscriptions, yet uses SMS rather than e-mail.
I currently plan to implement Clickatell, Mobile Accord, and mBILL. Other suggestions are always welcome on the SMS Framework group.

Comments
Susbcriptions not an API
So....there is moshe's "replies" module that he was hoping to get into core. I haven't checked into it any further.
No, subscriptions is not an API....but it needs to be. This may be more than you can tackle, but PLEASE don't write an entire subscriptions framework from scratch: put out a call that your SMS functionality needs this...and see what happens.