I have just been reading SketchFlow – Sketching and Prototyping in Expression Blend. This looks really cool, and I can hardly wait to get a copy. It has been a long time since I saw a new tool that looks like it is made to use on my Tablet PC. There is also a good writeup on SketchFlow by Loren Heiny here.
Right now, I do design in OneNote – but of course there is no connection to any other tools used in the implementation of the design.