You can send SMS messages to mobile phones by email. Every major carrier also has their own email domain.
Use the full 10 digit number, including the area code. And keep it to 160 characters. The T-Mobile works great. I use it to get text messages whenever the web server gets automatically restarted due to some problem. Works a little too well, if you know what I mean.
From Cingular/AT & T:
The above web page from Cingular/AT&T lets you enter in your friends phone numbers and then it tells you if they are part of the Cingular cellular network or not.
From Verizon Wireless:
The above web page from Verizon Wireless lets you enter in your friends phone numbers and then it tells you if they are part of the Verizon Wireless cellular network or not.