This video from PDC 2008 is interesting; Alex Mogilevsky talks about IE8's new layout engine. It was generous of him to mention that the subpixel units and zoom approach first appeared in Firefox! It's interesting to get a little glimpse under the hood. It sounds like they have something much like our frame display lists. The glimpse we got of their "layout object dump" and "display list dump" output was quite revealing if you know what you're looking for, but I'll leave that as an exercise for the reader :-).
On the flip side, the decision to make the layout code reusable for apps like Word seems dubious, and I fancy I could sense him doubting it too. And Alex seemed remarkably sanguine about supporting an IE8-mode in IE9; "putting an 'if' statement around every bug fix" is far from the whole truth and would itself be a big maintenance burden. I stand by my skepticism of their whole approach here.