I was running 3.1.2 JB'd Using Pwnage tool months ago. Attempted upgrade to iOS4 using Pwnage tool 4.01. Now I have no carrier! Everything looks correct - Cydia and all applications are there - Just no AT&T!!!
Well, it was hard to judge, but it certainly was dropping quite fast after I put ultra on. Either way, the answer was to re-pwn and not choose Activate!