Wow, just (2 yrs later) got around to trying out DJ's recommendation of Synergy2. (Moved house, new desk config, and never liked cabled KVM.) This is amazing. Scroll right (or left, up, down, all configurable), and your mouse - and hence your keyboard - are in the other screen/window. No obvious performance penalty (although I didn't try it over wifi

and best of all: copy and paste works between the 2 systems!!! Even when 1 is Ubuntu and the other is W2K! Truly useful, and worth the install alone.
Note Quick Synergy is also available as a separate GUI for Linux.
Thkx DJ!