Here's an idea for a plugin for Buddypress. I may try to write it myself in the near future.
Allow a user to send an sms message to another user via their profile.
After considering gateways, broken free plugins, etc, I reason that the best way to accomplish this is to use the carrier-provided email addresses that deliver SMS.
In the profile fields, add fields for carrier and number.
Probably would need a lookup table for carrier-email suffix lookup.
Then if a user goes to the profile page and that person has their carrier and number filled out, the user can send a message.