I think it should be 5" with a wacom touchscreen and stylus for collaborative note taking, come with an unlocked HSDPA sim card slot, support bluetooth and usb keyboards for note taking as well as come with a built-in table stand for when using the keyboards. It should be like the latest Samsung E-Ink prototype:
http://gottabemobile.com/2009/01/09/...s-e-ink-device
But in my opinion without any border around the screen. And have the minimum amount of buttons on the side. Only buttons needed next to the screen on the front of the device should be small next page, previous page, menu and other few shortcut buttons. This way, such a 5" E-Ink device could fit in any pocket.
In terms of software services, I think it should come with a totally open rss feed agregator and a way to select pages with your computer that then are beamed to you E-Ink reader so that you can then later read all of it on an E-Ink screen instead of a normal backlit computer screen.