Kindle can do notes
I have one (if it makes you feel better, it was a prize, I didn't buy it). You hit the Menu button, pick "Add a Note or Highlight," and go from there. It has a little keyboard that you can use to enter your note. Then you can use "My Notes and Marks" (next item down in the menu) to jump back to it later on.
Having said that, I think textbooks would have to be re-thought to work better as an etextbook. If that were done, I think high schoolers would love them — with lockers too tiny to be useful these days and too little time to get to them between classes, kids around here regularly carry a 30-lb knapsack full of textbooks most of the day.