Those possums sure do have that 'playing dead' thing down to an art. Twice in the last week our labs have encountered possums (one had 'shaken-baby syndrome as a result of capture, I was sure). The 'shaken baby' played dead as soon as my dog released it ---- he watched it for awhile and then walked away. It got up and left.
Then today, the other dog was standing over a lump of something barking continuously. I called him in and the lump just stayed there for almost an hour. I checked on it again and it was gone. Amazing.