Do you need to register upon your arrival in Poland? Any experience, guys?
The law says that you must register your place of residence within 4 days of entering the Republic of Poland. In practice, if you don't need a visa (this includes people who can enter on the tourist stamp) - you don't have to.
But you still have the 90 day limit to obey. I'm not sure why you're asking about registration when you can't register now anyway - you're illegally in the Schengen zone and cannot legalise your stay now.