Wow. excellent, enlightened idea. Having attended conferences, I would really have appreciated such a thing. I'd support everything the experts have been saying.
Assuming you are going to have conference materials on the device, could I make a suggestion? Get the sort of material you want to distribute and print it out on paper (black and white if you are thinking of e-ink, colour if LCD, et al.) of the sizes you are thinking of using. See how it feels to look at on the various sizes (for added authenticity, you could put black card between the pages
). The reason is you mentioned pictures -- depending on what they are how well they look may depend on the size of the display. I'd also recommend seeing them on a display of the relevant greyscale count. I find a massive difference between the Kindle and the iLiad thanks to the greyscale differences; but that doesn't matter on pure text or line-drawings that have been correctly rendered.
I'd have loved a scribble ability to place notes on or with material as it is being presented -- decide if you think people will do that, and so what sort of interfaces you want.
If you want people to use it during the sessions, think about the lighting you will have. Dark and e-ink don't necessarily go well together!
But I'm sure that whatever you choose, it will be a big hit. Go for it.