The major US cellular carriers use the 10_digit_number@cell.carrier_domain.com format for SMS to text capable cell phones, with a limit of 160 characters in the subject and message body (total).
Carrier Send Email to phonenumber@....
Alltel @message.alltel.com
Cingular @mobile.mycingular.com
Nextel @messaging.nextel.com
Sprint @messaging.sprintpcs.com
SunCom @tms.suncom.com
T-mobile @tmomail.net
VoiceStream @voicestream.net
Verizon @vtext.com

Verizon users can register an alias at vtext.com to give out instead of their phone number. Both alias@vtext.com and phone_number@vtext.com will be valid addresses for the phone.