Quite a few old MobileMe users are seeing this.
At the end of September our legacy 25GB storage limit was supposed to drop back to the free 5GB limit, unless we paid for more. Several people around the net are reporting that they are now seeing that date pushed back to the year 2050. Since there has been no letter or announcement from Apple the general conclusion is that it's a bug, or something.
I'd already decided I did not need 25GB, but wasn't comfortable with just 5GB, so I opted to pay $20 for the 15GB storage limit. When the news first started circulating I checked, and was still due to pay on schedule. I figured the bug missed me.
But my bill did not show up, so I looked again.
All I can say is that I hope I'm not too senile to remember to pay when the bill does come due.