I just override the gantry schema.css with percentages (for example div.rt-container div.rt-grid-4 {width: 30%;} =4/12). That works somehow but breaks every visual design-intention of Mr. Evans.
I think the day will come that Andy says "now let us make a fluid one" but till then ... stick to 960px ...
I have tried creating a fluid 960 and you should know that it's not possible to make a "perfect" one because all browsers handle percentages differently. You can download the fluid 960 grid at
www.designinfluences.com/fluid960gs
. Try it in different browsers and you will see that they don't always extend to the full width. The only way I found to make it work, which won't be consistent widths, is to figure out which is the last column, add up the width of all the other columns in the row, subtract from 100 and apply that width to the last column. I recommend floating that last column to the right so the grid margin absorbs the difference. It's far from perfect, but it's the only way. I have spent days doing tests on it.