I thought the rulebook stated that they are kept for the duration of the current game. (Page 9 - Changing Control)
I will change that in my rulebook if need be, Matt. Being from Cryptozoic, you would know how it should be and I will go by your word on it here. Was the book a misprint?
Oh, I forgot all about that rule. Very few games allow you to discard other people's cards into your own discard pile, so I totally forgot that it was spelled out the other way. Editing the above to avoid confusion...