Microsoft: Design guidance for Windows Store apps
Don’t be put off by ‘Windows Store apps’ in the title. There’s lots of really good information about designing for tablets in general here.
Aside: I spotted this in the ‘Windows 8 touch posture’ section:
Because slates are most often held along the side, the bottom corners and sides are ideal locations for interactive elements.
That’s a big difference between iPads and Microsoft’s Surface ‘slate’ (do they actually still call it that?). Microsoft sees the 16:9 Surface as a landscape device, whereas a 4:3 iPad is for use any way up.