OK. Procedure looks to be the same as for 10.10, so I've updated that thread.
Catalyst 10.11 has just been released:
More information to come. Release notes not available yet...
OK. Downloaded and trying a plain
$ ./ati-driver-installer-10-11-x86.x86_64.run --buildpkg Ubuntu/natty
Generated. Install? Why not.
Nope. dkms won't build, it'll need to be patched. Tsk, ioctl again!