I thought something might be a bit odd so I tried opening an unlayered, tagged TIFF into ACR from Bridge CS5. The Camera Raw option was greyed out. Tried it with a layered, tagged TIFF and Camera Raw was greyed out. Tried with a JPEG and the Camera Raw option was active.
Tried opening a layered, tagged TIFF into ACR directly from PS and it worked. It did flatten the file; however.
I then went to Bridge CS3 and tried the same two TIFFs. With the layered TIFF, the Camera Raw option was greyed out. With the unlayered TIFF, the Camera Raw option was active.
Perhaps something's amiss in Bridge CS5?