Let me try to give more information. The big problem with backlight control is 
that there are many ways to do it. There is a generic implementation in the 
acpi subsystem and then there are several vendor specific drivers that do the 
same with all the different interfaces vendors came up with.
With Acer laptops this driver is acer-wmi (And a Thinkpad uses thinkpad-acpi, 
so any feedback from non-Acer users is nicely meant but, sorry, totally 
useless).
Back in Intrepid (when it was working for you) both drivers (the generic and 
the vendor specific) were allowed to touch the backlight. But this caused 
problems for some (values changed twice on each keypress). To fix that some 
patches from upstream were backported that would only allow the acpi or the 
vendor driver but not both. Beside of other problems, this also did change the 
acer-wmi driver the wrong way. Now the driver would only work if the acpi 
generic driver was also there. But this bug is fixed in Jaunty.
So for Acer, and sorry if this is mostly duplicated, at some point long 
Launchpad bugs get confusing:
1. With the old Intrepid kernel that works, which directories are populated 
there?
    (/proc/acpi/video, /sys/devices/platform/acer-wmi/backlight, both?)
2. Then going to Jaunty (as this has the acer-wmi bug fixed at least), which of 
the places above are there.

Can you do a "grep -r . <dir>" and "ls -la <dir>" for the existing directories 
above (put all the output into one file and add comments which is from which 
kernel. Then attach it here as an uncompressed file, that is the most 
convenient form to look at).
The first thing that has to work would be the change when echoing into one of 
those interfaces. If that doesn't work, then the brightness applet cannot work.

-- 
Cannot change brightness with 2.6.27-11+ kernel
https://bugs.launchpad.net/bugs/333386
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