hmm, I stand, somewhat, corrected. Supposedly you can get iBooks with voiceover to auto-turn pages and continue reading by:
from macrumors.com forum post by user LeNex:
"1. Turn VoiceOver on. Unless you're using it all the time, I suggest going to the settings pane and choose Accessibility, VoiceOver then check 'triple click of home to turn VoiceOver on or off' or whatever the option is. Can't check right now as I am using my iPhone to post this and if I come out of Safari this text will disappear.
2. Open iBooks and the book you want. Triple click the home button to tum VoiceOver on. It will say 'VoiceOver on'.
3. Place two fingers on the screen. Make sure they're fingers held next to each other. Flick those fingers downwards in a smooth quick motion. You may have to practice to get it right.
4. VoiceOver will begin to read the page from the top, and when it gets to the bottom, it will turn the page for you and carry on speaking the next page. And so on. To stop it just tap on the screen anywhere with one finger (VoiceOver will most likely read whatever is under your finger, then stop. )
5. Unless you want to carry on using VoiceOver gsetures for everything else you do with your ipad, remember to triple click to turn it off again when done."
But, once the screen display goes off, I still think it will stop, as iBooks itself is idled (what's the point of keeping an ereader app fully active when the screen has gone blank?).