I had a magentic money clip which ate credit card strips if I dropped it onto the wrong pocket. Mind you, that was a long time ago, and it was a strong magent. I've always kept credit cards in the case next to my other XDAs without problems.
I've been using the Krussell case for the Wizard for a couple of months now, and it's great.
A really good clip and release mechanism, well crafted, and it now takes 3 credit cards after a bit of stretching. Easily the best case I've ever had.
XDA Mini S & DConnex Bluetooth GPS