It is not easy to treat migraines and every new medication is a big help. Until 1991 we did not really have any powerful drug for migraine headaches. The advent of Imitrex has changed that and we got a powerful weapon against migraine.
But since then there has been nothing new insight. Until recently. Botox maker Allergan announced on 9/11/2008, that the results of Botox for migraine treatment studies are promising. It turned out that the drug is mostly reducing the number of days that the patient has headaches.  The migraine episodes? frequency decline was not that impressive. 
The number of patients enrolled was 1,384 and they were treated with multiple injections around the forehead and neck. This method differs from the previously used method to inject where it hurts. And it apparently worked.
How does Botox work for migraines? There is no confirmed explanation, but I can only guess that Botox relaxes head and neck muscles, therefore relieving the pain. The most promising thing is that, as per Allergan, Botox treatment has helped people who are suffering for severe migraines, not light headache episodes. If it is true in everyday practice, then we may get a very powerful weapon against migraines very soon.