I been having this problem since last week...I reprogrammed my phone and everything and it didn't do a thing...When i called sprint TS they said that part of the system was down and they couldn't do anything for me and would get back to me...they never did but the woman told me that my number might have to be re-added to some list to be able to roam again...She said that someone else had called in with the same problem.
Check this thread out... seems its actually on verizon's end and not sprint's
http://forum.ppcgeeks.com/showthread.php?t=15923