I know it's been over a month since anyone posted here, but I thought I'd post anyway.
I received a 16 gig iPod Touch for Christmas. It has worked beautifully for the last six months. Then I purchased the same Griffin flip case, and soon noticed that it would start freezing up on me (I never made the connection between the case and the freezing until recently). It was fine as long as it was left sitting on the desk, but as soon as I would start walking, or even sitting in the car or bus, it would freeze, unable to turn it on. Doing a hard reboot of the device worked for a while, but after about a week I couldn't even reboot (nor would iTunes detect it when I attached it to the usb). I took it to Apple, and they replaced it.... but the very next day the new one started doing the same thing. After another week, it also stopped responding to the attempted reboots. This time I took it to apple, and iTunes could detect it, but it could not run the restore function on it. Again they replaced it. So far the new one is working great, but I haven't put it in the case yet. At this point, I'm not even 100% sure if it is the magnets, or just a bad coincidence. I'm planning on waiting for a few days (maybe a week) then placing it back in the case to see what happens.