Thanks for the very informative replies. So it seems the kindle and sony readers do it best.
In the meantime I've been watching some reviews of e-readers on youtube. Dictionary look-ups involve interactivity, and thus responsiveness of the device is important. This is a known weakness of e-ink based readers. So now I'm considering LCD based ones, that are hopefully more responsive.
I've been impressed by the Nook Color. Looks like the brightness can be set very low, and it has a night mode where text appears white on black instead of the opposite. Hopefully it might alleviate eyestrain problems.
What do you think of my reasoning ? Are LCD based e-book readers likely to perform better for dictionary look-ups than e-ink based devices ?