I've had a somewhat? similar issue that I haven't seen anywhere here., I jb too, and unless I activated 3G and maybe even left 3G antenna turned on BEFORE I jb, I could never get 3G service after I jb, only Edge.
but once I reset all to factory, made sure 3G was on, and then jb I always get 3G fine. If I did not do this, I always only got edge.
It was odd. works perfectly but only if I do the set up as indicated. I haven't found anyone else having this issue.
I had had a genius appointment after tech at Apple and ATT could not get 3G connecting, after lots of resets etc. later I starting toubleshooting myself, then I unjailbroke, reformatted and 3G worked fine. I cancelled my appt seeing as it was related to the jb. then I just turned on 3G first, it all works fine.