Hey, everybody. I actually attended the Amazon press conference today. (I said I was "from mobileread.com"; I hope you don't mind, Alex!) I took some pictures and some video, and got to handle the DX a little bit. I'll be starting another thread with some comments, pics, info, and links to movies, as soon as they finish uploading to Youtube.
In the meantime I'll say that it seems to me like a killer app. Amazon just totally raised the bar with this. I don't know about pdf highlighting/note-taking, but I can say that pdfs are totally readable even in portrait orientation. In landscape orientation, you're essentially seeing the pdf full-size, half a page at a time. The screen is 8"x5.5", which is pretty much half of a standard letter-sized page.
The DX is a bit heavy and feels quite solid. I'm not a huge fan of the buttons, but the keyboard is unobtrusive and seems to serve its purpose.