Many carriers support sending SMS by email to their customers, but not all carriers do this and the domains are different for each carrier.  For example, to send an SMS by email to Cingular customers, you’d use <phone_number>@cingularme.com.

Enter: TeleFlip.  The TeleFlip service is intended to get your email on any cell phone via SMS, which is fairly clever, but useless for me since I have a smartphone serviced by Exchange.  A feature of TeleFlip is the ability to send SMS via email for free to any cellphone, regardless of carrier, by using the TeleFlip service by emailing to <phone_number>@teleflip.com.   

I just tested it, works like a charm!  The only indication you used the TeleFlip service is the tag line at the end of your SMS message that says  “Email on your cell — teleflip.com” to advertise their service.