In reply to JimR:
You can roller it if you want. Yes it will hurt, and yes it will help, but eventually it will come back. The problem is, if you roller the ITB, you are trying to lengthen tissue that is already lengthened.
The reason for the pain is friction from the ITB as it is tightened over the pit of the knee where it hurts. Think about it like an elastic band. If it was tight, you wouldn't roll it out and stretch it more to fix it, would you? You'd slacken it off.
The Muscle that tightens the ITB is the TFL (tensor fascia latae) As pointed out above. if you roller the TFL, then the ITB *should* slacken off.
The reason for it being tight can be any number of about 7 or 8 possibilities, generally the most common seems to be weak glutes and lateral rotators (piriformis).
So, get a foam roller. See a physio, and if the physio only massages your ITB and doesnt look into WHY you are getting ITBS, ask them for an opinion, if they dont have an opinion, go to a better physio/osteo/massage therapist. And dont be talked into getting a pair of orthotics- Id only look at those as a very very very last resort.