I tried jailbreaking my iPhone on 1.1.2 firmware just after I got it and got this problem.
I eventually couldn't fix it and restored, whereby the problem persisted. I despaired for 2 days, then all of a sudden I got 2 days worth of texts through at once and the signal started coming through patchily, eventually returning to normal service. I thought it MAY have been something to do with the number of times I reactivated the phone having reinstalled the firmware, but never got to the bottom of the problem as it just fixed itself.