Comment 27 for bug 554569

It appears that the following upstream patch may be the fix:

b91ad0ec52770dcb622b94fc1f57e076686f427a drm/i915: Rework DPLL calculation parameters for Ironlake

Here's an upstream mailing list thread about it:
http://markmail.org/message/4yq3ejcke7uhe63i#query:+page:1+mid:ljfvjcr7or7yrd6n+state:results

I'll have a look and see about getting it into a test kernel, I'm busy at UDS this week