A lot of people I text have Nextel. So what I do is use their cell number email address (Like below) and it gets sent to them as a text message. It doesn't even get charged against my account as a text message. Other carriers have the same setup...
MAKE SURE your email is sent as TEXT not HTML. And if you use email to send the TXT you can send to as many people as you want.
XXX-XXX-XXXX is their actual cell phone #
Example
XXX-XXX-XXXX@messaging.nextel.com
I just tried this one and it sent my iPhone a text message...
xxxxxxxxxx@mobile.mycingular.com
I think this should work too
xxx-xxx-xxxx@mms.att.net
Here is a list of others too
http://www.livejournal.com/tools/textmessage.bml?mode=details