As it turned out, Apple has changed the way Game Center authenticates user in iOS 4.2. Due to this fact it is now only possible to submit score and achievements on fresh run of Portaball. It means that game must be closed from multitask menu and opened again in order for Game Center to work.
This is a really troubling problem. I'm really sorry I haven't checked release notes on Dev Beta of iOS 4.2, I should have done that. I've just submitted Portaball 1.2.1 which fixes this issue. Hopefully situation like this will never happen again.