This is very similar to a minimal grid system I developed a couple of years back, but I define the fractions on the child elements, rather than the parent, eliminating the need for extra classes. I don't make any assumptions about margins, but you could easily apply border-box and padding to the columns (like Ingrid does). I really like that border-box is picking up support. I find it a much easier way to think about the box model, because I find it applies well to UI development in HTML (especially on mobile devices like iPads).

GitHub Project: https://github.com/colingourlay/frak

Demo: http://colingourlay.github.com/frak/