On a mobile device you scroll a window, not the site
When you scroll on a mobile device, you are not really scrolling the site, in fact if you zoom out completely you will see the entire page, you are only scrolling your “window” around the site. When you understand this, your fixed footer dream really shatter, unless you are willing to use complex methods..
And there start your quest, on your way you will find iScroll 3.0. This little script create the illusion of a fixed footer, but the performance price is rather high. It emulates scrolling alright, but this is not quite as snappy as the real thing, a big problem if you consider that your users will have a feeling that something is not quite right.
There is no perfect solution
Personally, I do not see myself implementing these solutions. I, however, stumbled on an interesting solution, make the footer fade out when you scroll, and fade is when you stop. This is not perfect, far from it (it is to wonder why Apple is not implementing something themselves in Webkit..). But I found this was the best solution offered to me.
I am working on a plugin for the jQTouch moible framework, I will probably release it in the weeks to comes.