Over the last 6 months or so I've become mildly addicted to Sudoku.  As it turns out, a buddy of mine here at Microsoft, Stephen Toub has built a killer Sudoku application for the Tablet, released a bunch of source code and an amazing whitepaper that explains it all.  If you are at all interested in how .NET can enable some very intersting game development for interesting form factors, it is definitely worth checking out.  I love the app itself, and it has been really inspiring to see all the interesting logic/math behind maing the game work -- definitely worth checking out if you have some free time.