Apparently it is the same as sending an email, but you send it to the phone number via the Service providers email address here are some

T-Mobile: phonenumber@tmomail.net
Virgin Mobile: phonenumber@vmobl.com
Cingular: phonenumber@cingularme.com
Sprint: phonenumber@messaging.sprintpcs.com
Verizon: phonenumber@vtext.com
Nextel: phonenumber@messaging.nextel.com

Where phonenumber is a 10 digit phone number.