Camera raw provides its own color space when transfers to photoshop. Photoshop should ask if you want to keep the profile, throw it away or not to manage it.
I don't know if prophoto to srgb would be absolute, wouldn't that imply no color shift? To my knowledge you will get some shift no matter what color space you convert to.
I may be mis-using profile and space terms here, but I hope you get what I am saying.
There are three types of rendering available when converting from one profile to another. Actually I believe there's a fourth type, but I can't remember it, maybe it's Saturation. This has nothing to do with keeping or discarding.
Among the many many informative articles at John Paul Caponigro's site there is this nice graphic and explanation of rendering intents just below Soft Proofing.
FWIW if anyone still reads the Photoshop Forum.
Just bumping, because I can not believe that no one cares about this rendering question.