Hmmm… I typed up a big thing and then realized the whole thing is fluid. Fluid designs are one of those noble undertakings that rarely work out without a lot of thought and testing. Line lengths get too long or short, especially in multi-column designs and it tends to quickly become a mess of fringe cases. You may want to put some limits on the max and min width of the whole thing, but that’s a whole new can of browser support fun.
Enough “If I were you…”, on to answer your question, I like this technique fairly well if you can have a footer with a specified height – http://fortysevenmedia.com/blog/archives/making_your_footer_stay_put_with_css/
The gist is that you have a clearing div that pushes content to the bottom (clear: both), then a negative margin to pull the footer back up. So if you have enough content to fill the window vertically it will just appear at the bottom of that. If not then it appears at the bottom of the browser window.
Also, if you have no need of a footer, it can be a few pixels of background color and it should still force your containing div to go full height.
I hope it helps and have fun.