You can send a text message to almost any number, as long as it's text-enabled (i.e. most mobiles, and some landlines).

Get help with sending texts from Skype.

If you send a text to someone located in the USA, the carriers that support receiving Skype text messages are listed below:

Alltel, AT&T, Cellular One Dobson, Cellular South, Centennial, Cincinnati Bell, Nextel, Ntelos, Sprint, T-Mobile, Unicel, U.S. Cellular?, Verizon Wireless, Virgin Mobile USA.