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).
border-box is the only sane way to work with boxes... it's the way things should have been from the start. The mental gymnastics involved in box layout is so unnecessary :(
Even using the grid normally, it ends up using more horizontal space than on the viewport, forcing a horizontal scrollbar (Chrome 18).
Another problem: setting max-widths on units. Any proceeding units are handicapped by this and often just leave empty space to the right.
I also don't like the presence of three sets of classes when two would be appropriate. "ingrid" as the container, and "span-x" for each column division. And this is unavoidable, as you need to define "in-[parts]" in the container to even use the span-x classes. I understand that it's less work if you stick to equal parts, but this is not the majority use case in my experience.
I find the naming scheme awkward. "in-twos" and "in-thirds" instead of "in-halves" or "in-threes". Keep it consistent.
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All in all, I wouldn't recommend this to my mother.
I think the .unit class could be done away with if he used .ingrid > * in its place. Of course, that might mean rewriting some of the CSS to account for the change in specificity in that rule.
I believe this would have performance implications (css selectors are read "backwards" by browsers, so this would mean every element on the page is examined to see if it's inside a .ingrid class). Not always, but definitely could for a large/complex page, and since this is a framework that will be used in all sorts of situations it's best that the author stays on the conservative side.
GitHub Project: https://github.com/colingourlay/frak
Demo: http://colingourlay.github.com/frak/