I'm currently experiencing a strange problem with my n810.
After a certain amount of time (or an action unknown to me) screen rotation stops working.

When I click the letter icons in the advanced backlight applet simply nothing happens. If I'm currently in a rotated state I'm stuck there.

There is an even more strange way to reproduce this:
After a reboot, when I rotate the screen before Wifi is connected, it immediately stops working the moment I'm connected. If I wait until the connection is established rotation keeps working until it stops after some hours/days/whatever.

I've installed the following package versions:

osso-software-version-rx44-unlocked (1:5.2008.43-7)
xserver-xomap (1:1.3.99.0~git20070321-0osso20083101rotation)
kernel-diablo-flasher (2 2.6.21-200842maemo1rotationfast)

I hope somebody has an idea how to track this down...

[Edit: got confused with package versions...]

Re-Flash The Kernel, Go to My Applications, Settings, Slide Rotate Settings, and Click on Re-Install Xandar Support.

Hope This Helps

I've already tried this several times. Finally I purged all the sliderotate stuff because I thought it might conflict with the newer rotation packages. Nothing helped...

I am having the exact same problem. After a reboot rotate works, but after some time is just won't do anything anymore. The net result is, that I cannot change the last orientation anymore and may get stuck in portrait mode.

I installed it following the instructions on the wiki (i.e. not sliderotate)