I had a card case for my iphone and my cards would routinely get corrupted. I couldn't figure out why it was happening, there wasn't a magnet in my iPhone 4s at the time.

I eventually figured out that it was happening because I'd been routinely laying my phone on my iPad, and the magnets in the iPad would, over time, murder my cards.

The new phones will absolutely corrupt your cards, I don't care what Apple marketing says.