Quote from revcompgeek:
Mine was stuck like that too. I ended up getting a flathead screwdriver and carefully prying up on the edge of the logic board in the back and it came off with an awful sounding pop. It looks like the processor was attached to the heat sink with something that is probably thermal paste. It doesn't seem to be damaged, but I haven't been able to test it yet.
You will have to gently work the area next to the fan housing... the key is to do it gently, small movements at a time. If using a flat head, (which you should probably not) make sure not to damage the PCB