Hi David,
I can't comment on the iPad but re: revising on a Win7 tablet - do you use Journal, at all? It's my number one revision tool because from Word (or Scrivener etc...) you simply print to Journal as you would print to a printer and then you have an image of it to scribble and circle and make notes on with your stylus - just like a red pencil. The best part, for me, is that because it's an image, not the actual file, I can't make changes until I've analysed the whole document and made sure my revisions are warranted and balanced (obviously this is not for mere proofing.)

I do love the Win7 on-screen keyboard, but I will always be a 'red pencil' reviser - even on screen :)