I have the same problem on an eeePC 1215N, Ubuntu 13.04 32 bits.
I found a workaround to get maximum brightness back:
sudo setpci -s 00:02.0 f4.b=ff
It resets the maximum brightness to maximum.
If it does not work for you, try another PCI device (find the adress with lspci), like
sudo setpci -s 00:02.1 f4.b=ff
I have the same problem on an eeePC 1215N, Ubuntu 13.04 32 bits.
I found a workaround to get maximum brightness back:
sudo setpci -s 00:02.0 f4.b=ff
It resets the maximum brightness to maximum.
If it does not work for you, try another PCI device (find the adress with lspci), like
sudo setpci -s 00:02.1 f4.b=ff