If the battery is fully charged and you still can't get the key out you may need to take it to the dealer. I suggest you read through this:
http://www.caymanclub.net/cayman-com...nual-read.html
It will give you some of the idiosyncrasies of Porsche's; the key in the ignition on a dead battery is mentioned. However, I would think once it's fully charged, the key would come out.