I saw a news cast about this very subject. It is apparently happening to certain brands of washers. They hadn't found a permanent solution for it yet. One repair guy suggested leaving the door open after washing, but that didn't solve the problem for some folks. Some folks said the bleach or any other product didn't work on theirs either. It had something to do with the water leaking inside the door somehow. If I can find the news cast I'll link it. It was awhile ago, and I'm not sure if I can find it again.
I thought about getting a front loader, and decided to hold off until I could find more information about it. We have enough trouble with rust in our current washing machine, and didn't want to deal with mold or mildew on top of that.
Bluemoon