in the Center of Innovation for the Information Society (CICEI) at the University of Las Palmas de Gran Canaria (Spain), a block named Sending SMS block has been developed. This block allows to send SMSs to Moodle course participants.
Instructions of how to use it are in Modules and plugins database in moodle.org. There are versions for Moodle 1.6 and Moodle 1.9.
We hope you enjoy it. Any idea will be well received. We'll try to answer here any questions related to this block.