Can't enter from outside the EU on an ID card can you? Thought it had to be a passport.
Doesn't matter where you come from - for the purposes of entering into the EU/EEA/CH, a national ID card (issued by an EU/EEA/CH state) is good enough. And as I said - it's actually illegal to use a foreign document to identify yourself to the Polish State if you already possess a Polish ID card.
The old style ID booklets on the other hand were never good enough to enter with.
If you think about connecting flights - you could fly from JFK to FRA, transit there en route to POZ - you won't go through passport control in FRA, and as far as anyone's concerned in POZ, you've came on a flight from FRA.