First thing: look
-> here <- for an example of a well thought out user interface for dictionary lookup, it even supports search for words you type in. All that with the very limited UI of netronix eb-100(600), which is basically Cybook.
Dictionary lookup on cybook takes up to 15 button pushes. But cybook's lookup interface is very basic and user unfriendly, but this could be easily relieved by putting some invention in the UI design.
In the future I assume that most, if not all liseuses, will have a touchscreen interface with the capability of onscreen keyboard. And that will make dictionary lookup very straightforward and, above all, FAST. I can see it on my touchscreen PDA, one tap to lookup the word, another tap to return to the book.