Found an interesting page about fisheye views, applied to XUL menus. If you've seen the OS X dock, you'll know what a fisheye view is, but you may not have thought about the cunning metrics going on behind the scenes to make it feel natural. Exscade uses distance measurement only (as shown in the diagram), but there's a short paragraph covering velocity sensitive approaches. I couldn't get the demo to work... Bummer.
Fisheye views are a way of getting more Fitt's Law bang-for-your-buck, by artificially creating larger areas to aim at. They're especially useful for cramped or small interfaces - but I'm stretching my head to figure out how to apply this to PDA interfaces. There's no such thing as a hovering cursor on a PDA, you see. ;-)