Most, if not all mobile carries have “SMS Gateways” that an take in an email messages and deliver them to their customer as text messages. This works also with mms messages.
That seems simple doesn’t it! It is, if you know the carrier the recipient’s phone is on, along with their phone number. Below are a few I have found:
AT&T Wireless (Formerly Cingular): firstname.lastname@example.org
AT&T Wireless (Formerly Cingular): email@example.com
Sprint (PCS): firstname.lastname@example.org
Sprint (Nextel): email@example.com
I’ve found a larger List of SMS Gateways. This may be more up to date and consisting of a larger number of phone carriers.