Asus already have a Tablet PC available that can do this. It's the Asus Eee PC T91. The screen flips around and folds back down onto the keyboard. It has multi touch with palm rejection.
It runs Windows 7 and has 2 Gig of RAM so should manage sketchup fairly well.
I'm not suggesting it for major jobs, but certainly for quick onsite drawings and presentation and such it would be ideal.
And it's a normal laptop as well, without being locked into the Apple world if you don't want to be.