[CentOS] C6.2 on DELL E6520

Hi,

I've installed Centos 6.2 x86_64 on a DELL E6520 and nearly everything
works just fine. The only problem is that the system hangs after
entering "reboot". I can see that several services get stopped, but
after some seconds the screen is just black and nothing happens
anymore. The weird thing is that "poweroff" always works just fine.
Also, I should note that I'm using the NVIDIA binary graphics driver
and have disabled the "Optimus" feature in the BIOS. The problem with
"reboot" happens regardless if the NIVIDA driver is installed or not.
Any ideas?

Best regards,
Bernd