Peter Rysavy sez: While selecting with a pen is definitely fast and intuitive, the text editing experience on a tablet is still very lacking. ... only a Microsoft will ultimately bring a decent editing experience to the mass market.
I've investigated writing a Tablet PC editing add-in for Word. Unfortunately, ink-collectors must be created in the same process as the Window for which they collect. Thus, using COM Interop is insufficient; you get an exception when you try to add ink to a Word application instance. The next question is whether VSTO could be used to get the process. I don't have VSTO on my Tablet and it's kind of a longshot and even if you could collect ink in Word's window, you're miles from relating it to the underlying text. All of which is to say that I think Peter's right that it will take Microsoft to create a compelling ink editor for Word documents.