Hi Newmoon, as Yamanboori says ACR will open via bridge or PS, both independantly ie you can only open Bridge and not PS to use it or both etc etc. I use it with my 645D and it is a very good raw converter, but i haven't used any others as ACR i feel does a great job, it is the same raw converter that is in LR 3, just LR3 uses or calls the sliders something different, but to all intents and purposes they do exactly the same job with the image.
i was told that Capture one was slightly better in particular in rescuing any blown highlights, you may have in a shot, by a user that has both and shoots a Phase.
i guess i can understand why they wouldn't support the 645D.
Happy shooting and PP.