At solutiongrove.com I wrote a very simple smsalert package and integrated it into bug tracker.
The mechansim provides a big red button for each ticket. When a client clicks on it, an sms message is sent to all dev team members. The message contains the project name the ticket number.
It's used by our clients for emergencies when a particular ticket needs immediate and urgent attention.
The SMS service provider we used is http://www.truesenses.com/ but there are other service providers like http://clickatell.com/ that also provide some sort of api that you can build on.
The nice thing about truesenses is that it provides an HTTP interface to send text messages. More details -> http://www.truesenses.com/website/pages/help/#How%20do%20I%20use%20the%20HTTP(s)%20Gateway?
Of course sending is different from receiving SMS and processing it say into a forum message. For that kind of thing, you might want to look into "Two Way Messaging"-> http://www.clickatell.com/brochure/products/two_way.php
