I believe the Kobo has a 600 × 800 pixels resolution screen.
At least that's the only pixel resolution I have have seen quoted in various articles and wiki pages.
Sources quoting 600x800:
EDIT: Thanks RR. I was thinking about this post as I was searching for credible links regarding the resolution and it hit me that it didn't really make sense... 800x600 would have been bigger than what Kovid has it set to now, so it wouldn't be cutting stuff off, doh!