I state this as a question because I'm seeing different behavior on different ~amd64 machines (as usual ;)
After upgrading from vbox-4.3.20-r1 to 4.3.24 yesterday I find that my linux guests have no 3D acceleration. Downgrading Virtualbox to 4.3.20-r1 fixes the problem on one ~amd64 machine but, of course, not the other one <sigh>. I'm still playing with downgrading mesa, etc, on the still-broken machine to see what happens. Anyone else noticing similar problems?
