Just upgraded from 12.10 to 13.04 on Dell xps13 ultrabook and the display is very dim and cannot be changed using f'n keys or from system settings. This ultrabook uses Intel's ivybridge mobile.
UPDATE: With the intention of reloading 12.04 I had to change the boot params to boot from usb stick. So:
- secure boot: disabled
- load legacy rom option: enabled
- boot list option: UEFI
- UEFI boot:
Must have not quite got params right since it booted from the ssd drive BUT this time display was normal and could up/down brightness? So need to investigate this more by playing with boot params but haven't time or patience now.