I think what you're referring to is a Sprint specific problem. Text messaging is Sprint's biggest fallback. I had the same issue with texting across time zones. Seems as if Sprint's SMS server time stamps the messages rather than letting the devices do it. So if you send a message from one time zone to another, they show as if they are an hour off. Example, ME: Hey 3:12PM, CHRIS: What's up? 2:14PM. So it appears that the other person has sent the message an hour before I began texting them.
I never had this problem with other carriers, and I think the same thing is happening when you experience your delay. And the reason that you may not have noticed before is because Sprint doesn't seem to leave the SMS service alone for long, they are always making wierd changes. If the delay is at the server end, it's giving an incorrect time stamp instead of relying on the sending or recieving devices to time stamp the messages as recieved. And of course, texting delays with Sprint is less about signal strength and more about the Sprint service. Windows mobile won't resort the messages based on real time since the message time stamps are off.
Sprint's text messaging is buggy. This is the only real complaint I've had with Sprint since I left Verizon 2 months ago, but easy to live with since I have half the cell bill I used to.
As long as I actually can SEND / RECIEVE, I'm content!
Texting quirks aside: Viva la Sprint!
Hope that answers your questions.
EDIT: Forgot to ask, what area of the country are you experiencing the problem?