This is actually rather easy to accomplish...
Most cell phone providers offer text-messaging through emails. For example, if you have a verizon phone with the number 555-234-1111, you can be text messaged at
555-234-1111@vtext.com. So adding a text-messaging option to a forum could really be as simple as telling your users to enter in that information as their email address, if they wish to recieve text-message notification.
However, if you wanted to make it separate from the email field, you would just need to duplicate the email and opt-in forms, and add another mail-check upon the posting of a reply. This is probably what you want, and maybe I will make it if I have some time, but it is a rather menial modification...