Today we released the source code for the demo I showed in my MIX talk, so you can try this for yourself using the Windows Phone emulator.
This is basically a mash-up of existing samples (I borrowed pieces of Skinned Model, Custom Model Effect, Generated Geometry, Simple Animation, and Sprite Effects), designed to showcase the five built-in effects on Windows Phone (BasicEffect, DualTextureEffect, AlphaTestEffect, SkinnedEffect, and EnvironmentMapEffect). It has a simple touch UI and some rendertarget transition effects, not to mention the all-important cat particles :-)
Along with this comes a new Snow Shovel mini-game, plus the Primitives and Primitives 3D samples updated for Game Studio 4.0 and Windows Phone.