Update Sept 2016: Unfortunately iOS10 no longer supports fixed positioning. A different way to approximate fixed positioning in email is using this “Faux” Fixed technique.
Its one of the least understood CSS styles in email and also one of the most quirky – especially when it comes to iOS. The iOS Mail toolbars will obscure fixed positioning elements set to the very edge and z-indexes don’t work reliably. However with patience you can do some cool things with fixed positioning.
Find out the various quirks of fixed positioning as well as a cool Taco Bell email example at the Email on Acid Blog.