Paulo's blog mentioned about this MSDN magazine article on Windows Mobile Gestures by Marcus Perryman. The article explains about gestures, the design considerations that went in while implementing it, the physics engine and a few tips and tricks. And don't miss the two part series, Let's talk about Touch, by Marcus Perryman, its a good read too.
Links
Going Places - Gesture Magic
Let's talk about Touch - Part I
Let's talk about Touch - Part II