Two of the CSS properties most lamented by mobile website developers is fixed positioning and scrolling overflow. These are absent for a reason. Poorly implemented, they might single-handedly render a website unusable on a small screen. (Imagine a giant fixed position header that hides all the content, or being zoomed into a small scrollable area and now you can't scroll the rest of the site.)
Johan Brook notes that these are now present in Mobile Safari in iOS 5 Beta 2, with a proprietary property and vendor extension. Will be interesting to see how they solved the potential pitfalls.
Update: the original article link is dead now, but this snippet covers how it works.