I have dell xps 13 too and the latest kernel where brightness works is 3.8.0-29. So I'm staying on it right now (Ubuntu 13.04) and according the comments it's not safe to upgrade to 13.10.
I have dell xps 13 too and the latest kernel where brightness works is 3.8.0-29. So I'm staying on it right now (Ubuntu 13.04) and according the comments it's not safe to upgrade to 13.10.