Regarding the "if they give out PDF, it will be on eBay illegally", I'm not quite sure about this. There are quite a number of publisher that sell pdf version of their book.
Being a book writer my self (in a 3rd world country), the biggest problem a publisher face is :
a) university student fotocopy the whole book. if the book is aimed at university student, this will hurt the sales.
b) scanned / pdf'ed version of the book floating online.
In the end it just like software. You can put any security you want, but in the end it will get hacked anyway. The question is you want to make it easy for your user or the hacker?
With increasing e-book reader sales, I think publisher will pick this up. Trust me, I read a lot of book and own a lot of book. My biggest problem is when I move, and also room space. If a book is distributed on a CD (or server download), I will be easy on both.
To take for example is music. At first nobody want to sell it online, thinking the music will be distributed - only to realize it will get distributed anyway. But then mp3 player sales soar, and everybody has download option.
The same thing will happen to books. Trust me.
http://www.fablefox.com <-- book writer, game programmer.