I had the same problem after installing and jailbrek iOS 4.1 on iphone 4 with original sim from 3 network UK.
The problem was that in order to downgrade from 4.2.1 to 4.1 i had to use tinyumbrella and to start TSS server.
What you have to do is after jailbrek you have to STOP TSS server or just quite tinyumbrella.
After duing that iTunes activating your phone and jobe done.
After activating you can use limera1n to jailbrek your phone.