That is an excellent approach! I have played with completely "relative to font size" layouts in the past and been very happy with the result. The entire layout degrades nicely and no unexpected surprises or drawbacks. Whatever "glitches" such a layout may experience are in my experience lesser than anything a fixed layout might experience.
The only problem, however, with the relatively sized layout is graphics. Not resizing the graphics may look quite unproportional. Resizing them may, of course, look quite pixelated. While I can live with pixelation for images (hey, if you need the font size larger, chances are you need my images larger as well) the immediate drawback is inability to resize the background images.
Thus, so far, I have not actually employed a "sized relative to font size" type layout ... yet. But, it is still in the works. Once I am satisfied with it, and once a few glitches are worked out, I'll be sure to post it here (and in CSS).