This specific Mobile Safari (seemingly impossible and yet undocumented) problem kept me going for a long long time today, and I was just about to post a question about it he
As someone with vision problems I'm heavily reliant on being able to zoom mobile sites. Now with the update to iOS 5 there's no way of getting iOS to ignore the user-scalable setting anymore and I suddenly discover I can't use half the web sites I use regularly anymore because I can't zoom them.
So, on behalf of people with vision problems, I beg you to please not use viewport meta tags to restrict peoples' ability to zoom your site!