It's now possible to extend your Facebook applications to accommodate mobile users. Facebook has released new additions to their API that allow you to specify FBML that will only be displayed on mobile devices and also allow for sending and receiving SMS messages.

Thankfully there are many restrictions on how an application can send you SMS messages in order to help avoid spam. You can read the details of the new functionality on the Facebook Developer Wiki.

I don't have a mobile device to test this with, but I would be interested to hear if anyone else has tried it out.