I just tried with the Oneiric final CD (kernel 3.0.0-12-generic), and had the exact same problems, i.e.:
* with the default boot options the laptop brightness keys made the brightness indicator window show, but the brightness wasn't actually changed, and
* with acpi_osi=Linux in the isolinux command line, the brightness keys were indeed modifying the brightness, but the indicator window didn't appear.
I just tried with the Oneiric final CD (kernel 3.0.0-12-generic), and had the exact same problems, i.e.:
* with the default boot options the laptop brightness keys made the brightness indicator window show, but the brightness wasn't actually changed, and
* with acpi_osi=Linux in the isolinux command line, the brightness keys were indeed modifying the brightness, but the indicator window didn't appear.