I have an HP laptop with dual AMD graphics cards (radeon HD 6520g + HD 6750M)
running Ubuntu 13.10 beta1, kernel 3.11.0-7-generic amd64.
This command
# echo "OFF" > /sys/kernel/debug/vgaswitcheroo/switch
throws a WARNING in the kernel logs, after which the folder
/sys/kernel/debug/vgaswitcheroo/ and the dGPU disappear (as noted in the logs)
(and power usage is notably lowered, guessing from the temperatures).

dmesg: http://pastebin.com/Pb7ELyqP , lines 1196+

Someone else in the #radeon channel also found this report:
http://lists.freedesktop.org/archives/dri-devel/2013-September/045157.html
which links to another old report from the same person.

Thanks.