Blog Kindle has one of the first reviews I've come across of the Kindle DX. The main thing that concerns me is PDF support. Here is what they had to say:
Kindle DX relies on it’s large screen to display PDF files “as is”, without re-flowing the text (which would be next to impossible with PDF since the format lacks any concept of paragraphs or text continuity). The only way to zoom that I could find is to switch to landscape mode. It’s not such a big problem because most PDF files that people would want to read are preformatted for either Letter or A4 page size and Kindle DX screen is comparable in size to these formats.
Although there is concept of pages in PDF and you can navigate to any given page, both internal and external links in PDF files are disabled. Structured table of contents that is present in some PDF files is not usable either.
Graphically PDF files look fine and crisp. Rendering time is also good. It usually takes around 5 seconds to open the file initially and after that pagination speed is the same as when reading ebooks.
Overall, the conclusion seems to be that if you don't need PDF support, stick with the Kindle 2. I'm guessing this may change as Amazon brings out textbooks for the Kindle DX.