I was issued a Residency Permit in Sept 05 (at a cost of 800RMB - some schools pay this, mine did not and didn't mention it before I signed up, either). The Z visa was cancelled. It most definitely has an expiry date - the last day of my contract. It is my understanding that the residency permit is specific to the job and if you change jobs you need a new one.

I confirm that the permit permits re-entry and that you can re-enter on a one-way airline ticket as I have just done this... tho you may get challenged on this when checking in for your flight.