Thanks for your hints.
I tried ati-drivers-11.5-r1 form the overlay and it did compile, but without changing the matter.
FWIW, here's my dmesg up to the crash.
IMHO Nothing exciting, fglrx module loaded without complaints.
[ 0.000000] Initializing cgroup subsys cpu
[ 0.000000] Linux version 2.6 ...