You can use a program called Easy SMS if your phone has BT or IR. Or if you have wi-fi access send an email using this cheat sheet by carrier.
SMS by Carrier
10digitcellnumber@vtext.com (Verizon),
10digitcellnumber@tmomail.net (T-Mobile),
10digitcellnumber@mobile.mycingular.com (Cingular),
10digitcellnumber@mobile.att.net (AT&T),
10digitcellnumber@voicestream.net (Voicestream),
10digitcellnumber@messaging.nextel.com (Nextel),
10digitcellnumber@messaging.sprintpcs.com (Sprint PCS)