It's probably reported before, but here it is again.
A bricked 1.1.1, downgraded to 1.0.2 and jailbraked to 1.1.1
If you boot with an AT&T sim it shows the bad IMEI
If you boot without a SIM it shows the good IMEI
Then insert the SIM, it still shows the good IMEI
Don't know if it can be of any use.