On my new M4600 I managed to install all drivers for the system. However, I noticed that after the Windows enters graphic mode (login screen), I am unable to change the LCD panel brightness.
The change display brightness slider in Windows 7 control panel does nothing. It is simply ignored. If I press Fn+Up or Down, the slider moves, but the brightness does not change. The Dell brightness OSD is displayed with the correct brigtness value, but it does not change (if I set the brightness to 50% before login screen at startup, this status displays 50% all the time).
I even tried installing official NVidia drivers, but the result is the same as with the Dell drivers.