I ported my number from Sprint to Cingular. Strangely, I didn't get a spam message. A friend of mine on Sprint did though.
My guess is that they're using the sprint website. You can send a sprint phone a text message by using a regular email client and sending the message to
yournumber@messaging.sprintpcs.com.
My guess is that they're just emailing every possible number Sprint could have. The great thing for them is that they really didn't even have to do anything differently within their spam programs.