It is possible to keep activation after updating to 3.1.2 if you follow
these instructions.
Specifically, the key appears to be the upgrade from pwned 2.2.1 to 3.1. Then the update to 3.1.2 is just a single click in iTunes. Of course, if you decide to follow my steps, you may skip 3.1 and just go directly to 3.1.2.
I just wanted to confirm that my method allows to keep activation with subsequent updates.