Kirk,
I'm having a very similar problem. The only difference is that in my case, my .xmp sidecar files are no longer recognized by ACR when I change (or simply remove) the previously selected Color Class for a particular photo. For example, if a photo has a Yellow tag and then I process it in normally with ACR, the xmp file is no longer recognized if I change or remove the Color Class tag at a later time.
I'm using...
ACR 6.7
CS 5
PM 4.6.8
Thanks!