I believe the freshest idea is unrelated to vasodilation. That classical
migraine with aura is related to chemical disturbance in the striate cortex,
creating a "spreading depression" which releases excess neurochemicals such as
glutamate and potassium in a wave across the cortex until these powerful
irritants reach the meninges, which is their first exposure to pain sensors.
I have much more information on this, but don't have much time to spend on it
unless there are some serious, motivated researchers out there who are
interested. Or try a google-search using some or all of the terms above, and
see who else is into this approach.