This question has been asked many times for Ubuntu 11.10 but it always has been closed and linked with this link, but that accepted answer is from 2010 and that setting doesn't work anymore for Ubuntu 11.10. Furthermore, the brightness setting doesn't exist anymore in "Energy" options. It is now on "Screen" options and it doesn't seem to remember the setting after reboot.
So, how to make Unity remember brightness settings?
Best Answer
The short answer is: you can't. Gnome power manager doesn't save the brightness setting. The best you can do is to automatically set it to a reasonable default at startup.
For 11.10, the only solution I have found that works is the one given here: http://blog.ishans.info/2011/09/25/set-brightness-automatically-at-the-startup-in-linux/
Save http://ishans.info/attachments/article/65/setBrightness.py locally, change the brightness percentage in the last line to your preferred value, and add a new startup application to run:
python /wherever-you-saved-the-script/setBrightness.py