Brilliant. What's important, with simply conditional CSS it's still usable with older IEs (ie. not totally broken). The collapsing functionality can be considered as optional usability.
I don't know...IE9 isn't close to being released is it?
It's about to enter public beta... release is probably at least a year out.
You'd have to browser-check and then load something like the jsTree stuff if it failed. That'd be a major pain.
But the performance and styling is looking really good. It's very tempting.