Update Sept 2016: Unfortunately iOS10 no longer supports fixed positioning
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.