The programming team has been aware of this problem since the beginning of the year, but I wanted to post it here as well as a warning to other users.
Applying an XMP update on a TIF file in PM will destroy any Adobe Camera Raw settings embedded in that file.
Mac OS X 10.8.2
PM 4.9 (final) and 5 (build 13764)
Adobe Camera Raw 7.2.0.46
To recreate the problem:
Open a JPG file into Adobe Camera Raw. Change the settings around. Click 'Done'. Return to Photo Mechanic to set the star rating, etc. Reopen in Camera Raw. On my system, the settings are unchanged. Now, from Camera Raw, export to a TIF. Open that TIF into Camera Raw and make changes to the settings. Click 'Done'. Adjust metadata in PM. Reopen in Camera Raw. On my system, all the settings have disappeared.
This is a big issue for my (scanning) workflow, and hope a solution is released soon.