Amazon.com has positioned the Kindle DX as being great for textbooks. That was clear from their product announcement. It's not the only game in town, however, and CourseSmart has developed an iPhone app which makes its 7,000+ textbooks now available on a teeny-weeny screen.
That's a big (no pun intended) issue, one would think, with trying to read a textbook on an iPhone. To be honest, that's why the Kindle DX was developed; even the original Kindle / Kindle 2's screen was not big enough.
Besides the Kindle and Kindle DX, Amazon.com has its own e-book app as well. Both Amazon's and CourseSmart's iPhone apps are free.
This is probably more of an addendum than a replacement for a Kindle or Kindle DX or a full-sized textbook. Besides the iPhone app, you can also download a version for your PC or view textbooks online.
I'm still not convinced e-books will replace textbooks, but I suppose it will depend on a number of factors:
The idea of reading e-books on a mobile device's small screen is not at all appealing to some, but to others, mobility conquers all. We'll see if textbooks, in particular, ever expand beyond a niche market.