One way which I've preached a few times is to use absolute positioning to make it fluid. It may seem contradictory, but it works for me.
What I would do is position it's left side to whatever the left column's width is, and position it's right side to whatever the right column's width is.
So, it'll look like this:
[code:1:5ff1840d1f]
#centerColumn {
position: absolute;
left: 144px;
right: 120px;
}
[/code:1:5ff1840d1f]
Hope this helps some.