I think you don't understand me. When I make changes on a NEF file in Adobe Camera Raw (like white balance, noise reduction, exposure etc) these settings are stored in an XMP sidecar file. When I later use PM to add or edit IPTC data, it writes these data in the XMP sidecar file also, but deletes the ACR data that were already in the file.
(I suppose this has happened already for many years, but due to changes in my work and the increased possibilities to make significant and reversible edits to RAW files, I find myself much more often than before reverting to my NEF files. The funny thing is, I noticed the same problem in PhaseOne Media Pro about six months ago. After I noticed it to PhaseOne, they fixed it in their next update.)
(PM 4.6.9 on Intel Mac, 10.6.8, Photoshop CS5)