Update:
the one-click install now provides the tools necessary for flashing the kernel in device.
You don't need a desktop PC to flash the kernel anymore (thank fanoush for the tools http://fanoush.wz.cz/maemo/ ).
Install the package, exit Application manager, run xterm:
sudo gainroot
/root/install-xrandr-kernel

this will install the RandR enabled kernel and xserver , as well as modify mce.ini to disable PowerKeyShortAction to only rotate the screen, instead of giving the power menu.

answer y or yes to couple of questions an voilá, rotating N8X0.

Use power key to rotate the screen (N800/N810)
Open the keyboard (N810) to rotate back to landscape, close it to return to the
previous orientation.

As always: REMEMBER TO TAKE A BACKUP FIRST!!!!!!!!!!!
It has worked flawlessly for more than 10 test installations on both N800 and N810, but you never know.