Gareth Heyes has posted about some bypasses that he's found for getting around the XSS Auditor functionality in some browsers:
I had a look at XSS Auditor for a bit of fun because Mario said it’s getting harder to bypass. Hmmm I don’t agree. I seem to remember the same flaws are present from the last time I checked it with a little variation. It is also a very limited XSS filter not supporting detection of script based attacks (very common).
He includes three of his own bypasses - using a "formaction" on the submit input in a form, using "target" to override the iframe external resource restriction and the injection of a specially placed anchor tag. Each of these comes with a proof-of-concept example and another is also included courtesy of Mario Heiderich.