Hi, I have a Bell 3GS but it's on 4.0 jailbroken with jailbreakme.com and unlocked with ultrasn0w on BB 5.13.04. I experienced no difference between when it was locked compared to now unlocked. I don't have a solution to your problem but it could be the iPad baseband you upgraded to. 6.15.
If you don't depend on an unlock ie your still on the Bell network with a Bell sim card, Try uninstalling ultrasn0w from Cydia and see if that helps if not then my guess its the 6.15 ipad baseband.
Hope this helped a bit