Recently I bought a Surface Pro tablette. I've never owned any tablette because I've never found them useful except for browsing internet and getting my mails, which I can do on my desktop, iPod or phone already. The only reason why I've bought it is because it is a computer just like a laptop than can be handled like a tablette. The best thing about this is that I can work on DSS when I'm away from home.
So I've been playing with it while being away and worked out the windows touch API to implemented it in my game.
Here's the result:
2 fingers to "grab" the grid for translation, zoom and rotation on Z axis. 3 fingers to rotate on X/Y axis.
It might not be the most efficient way to zoom in as you can achieve faster zoom with the mouse wheel. But it is very intuitive, you just grab the grid and manipulate it like you wish. Also, I was thinking to implement double tap for quick zoom in.