I've been told this works on ppc, and it will probably work on amd64 as well. Minor graphical glitches (especially with scrollbars) are expected.
~ppc
I've built this on AMD64 now (edited the .ebuild to add amd64--didn't know another way to do it). It seems to work--gaim gets styled. Firefox doesn't seem to, but I don't know if that's normal or not.
I already marked x11-themes/gtk-qt-engine-0.6 ~amd64 in response to bug 70250.