It's not a well-known fact, but Apple allows developers to apply for an expedited review, should there be a valid reason. Since it wasn't me who changed iOS API, I didn't hesitate to do so. It worked! I've received nice e-mail from review team that they've made 'one-time exception' and Portaball was rushed into review.
As a result, Portaball 1.2.1 version is now available and hopefully fixes Game Center problems.