Sending a text from and email account is quite simply. Simply find the email suffix for the various carriers:
vtext for Verizon,
txt.att.net for the "New att",
tmomail.net for T-Mobile,
messaging.sprintpcs.com for Sprint, etc.
So a message to your friend in San Jose using T-Mobile would look like 4085554321@tmomailnet.
With the recent name change that Cingular is undergoing, the suffix "cingularme" is beginning to get phased out. Messages are still being sent, though with varing degrees of latency. Sometimes a text can take up to 6 hours to transmit.
Make sure that any automated mail text messages to att subscribers are updated to reflect the "txt.att.net" suffix. I suspect that "cingularme" will soon be turned off for good. If you are experiencing problems or latency for email to text messages to a particular carrier's customers, it is a good idea to check to see if the carrier has changed their text suffix. The carriers do change these suffixes from time to time.