This example demonstrates a fluid css menu which uses percentages widths for the menu items. It doesn't have any gaps at any width and works on all current browsers (Jan 2009).
IE6 has a 3px gap between the last two elements, because that's what IE6 does between floated and unfloated blocks. If you want to fix that gap, you can add a negative margin to the second last element:
Personally, I think IE6 can go get fucked.