I had the same question before the first time I flew with an ebook reader. I've flown with my PocketPro and my Cybook Gen3 and never been asked to remove either one from my bag.
The last couple of flights I've taken, the flight attendants made a point of saying "Anything with an off button needs to be OFF." To me, that means completely powering down my smartphone and PocketPro. I read the in-flight magazine or do tatting until the plane reaches altitude.
Once I'm reading it, as Bob already said, the flight attendants tend to be fascinated with it (also with my tatting) and ask lots of questions, so be prepared with your Ebook Evangelization Spiel!
The last time I flew, I had a round tube with quarters and pennies in it for souvenir pressed penny machines, and I forgot to take it out of my bag before putting it through the x-ray. They pulled me out of line to do a full search of my suitcase, but they just went through it looking for the roll of coins, which no doubt had a suspicious appearance on the x-ray, and never said a word about my PP.