I've never seen or heard of this occur on my Transform (as well as 3 other Transform users that I know of) or other people that I know of who have other Sprint phones. However, a quick research on the web regarding the subject of text messages (SMS) being split up shows that there is a 160 character limit for Short Message Service (SMS) since its inception by design! Thus, what you are seeing is probably a very long text message that Sprint (Verizon, AT&T, and T-Mobile do the same thing) has broken up into two or more text messages. However, if it's splitting it up a text that is less than 160 characters, then there is an issue with Sprint in processing the text message to your phone though it may be a random occurrence or we would be seeing or hearing more issues about this.
Update: I've read that some users of Handcent or GO SMS have reported issues in the past wherein their text messages where split randomly (3, 4, or 5 parts) even though their text message was less than 160 characters so it could be a Handcent or GO SMS app issue and not a Sprint issue. Btw, I use stock SMS and pretty much everyone I know does too.