craftgrrl | Cell Phones and magnetic strips?

It's odd, but it pertains to craftyness.

I want to make a cozy/pouch for my cell phone, and then I thought, hey, why not make it just a tiny bit bigger with a little pocket for my id & card for when I just want to run out?

Then I thought, wait a minute...there's a battery/energy source on the phone, and I'm not sure if that would generate enough force to wipe out the magnetic strip on the card. (I think it's magnetic. Or if you get it around magnets it will erase it or something. One of those things I learned once then promptly forgot.)

Does anyone know if that's a possibility? If I put my phone on my credit card, will it erase it? Or am I just smoking crack? Should I just go aheadn and make it, being sure to put an extra layer of something in there to prevent that from happening?

Can anybody (physics major, Visa representative, phone technician, hobo, whatever...) give me any insight?