Found a simple way, each cell provider has an email extension to use to message people:
Alltel
[10-digit phone number]@message.alltel.com
Example:
[email protected]
AT&T (formerly Cingular)
[10-digit phone number]@txt.att.net
Example:
[email protected]
Boost Mobile
[10-digit phone number]@myboostmobile.com
Example:
[email protected]
Nextel (now part of Sprint Nextel)
[10-digit telephone number]@messaging.nextel.com
Example:
[email protected]
Sprint PCS (now Sprint Nextel)
[10-digit phone number]@messaging.sprintpcs.com
Example:
[email protected]
T-Mobile
[10-digit phone number]@tmomail.net
Example:
[email protected]
Verizon
[10-digit phone number]@vtext.com
Example:
[email protected]
Virgin Mobile USA
[10-digit phone number]@vmobl.com
Example:
[email protected]