I am a consultant for several airlines and business jet operators. We have discussed this questions more than 18 months ago.
We have now with one operator several manuals transfered to E-Book Reader (Sony 505, already the Kindle) and are very happy. Pilots like it, no technical problems, long battery life, easy to update, good readable in the cockpit.
We have a workshop next week, where most likely some of my other customers are changign over also.
Our local authority gave a simple "Go Ahead", when we showed three things:
1. No electrical interference (we have disabled any wireless functions).
2. Data protection (USB connection secured).
3. Backup battery must be reachable.
I just can recommend to everybody to look on it.