What's not very clear from Jon's 3D box model is that the border exists around (outside) the padding and background image/colour. As far as I recall it also does not overlap the margin.

Stacking order is another thing IE screws up unfortunately. Borders overlap content in IE - which can normally only be seen if you style to force the content to shift over onto a border. I had a really slick rounded rectangle header tab style created using the left border property - until I discovered it didn't work in IE and had to resort to something not slick at all.

Any hope the W3C will completely redo the currently non-sensical box model with regards to width and height properties? Probably not, but one can always hope.