Karsten,
Wow - sounds like a lot of confusion here...
First of all, any image adjustment changes made in ACR that aren't actually written as a JPEG preview into your NEF files (can I say Hell Freezing Over?) aren't going to be shown in Photo Mechanic. ACR writes "non-destructive" edits to NEF files into XMP sidecar files so that ACR (and probably ACR only) can "render" those changes. Naturally Photoshop will show these changes as well (since they are Adobe proprietary changes to RAW files saved in XMP).
Now, regarding IPTC, this is a different story (and a big mess thanks to most other software besides Photoshop and Photo Mechanic).
However, if you choose to embed XMP into a (TIFF-based) RAW file, then you will likely experience problems with an Adobe workflow. The reason (as we have mentioned before) is that Adobe READS from embedded XMP but only UPDATES the XMP sidecar file. This means that if you make certain adjustments to a RAW file using ACR/Bridge, only the XMP sidecar file wil be updated, and therefore when you look at the same file later, it will still have the (stale) settings in the embedded XMP. This could be considered a "bug" in ACR/Bridge: if it only writes/updates XMP sidecar files, then it should use those as the master. So until Adobe fixes this problem, you should uncheck the option to embed XMP into RAW files.
Now regarding embedding IPTC in RAW files, Adobe's products don't have any problems with this (although this isn't the case with several other software programs or operating systems such as Apple's). So if you are using an Adobe only workflow (i.e. no Aperture or C1 etc), my suggestion is to go ahead and embed IPTC (but not XMP) into RAW files so that Adobe can retain your captions (along with other software that can handle embedded IPTC info). It is up to you to decide whether or not you should always create XMP sidecar files, etc, when updating RAW files (e.g. do you mind that an XMP sidecar file is always created with only IPTC info when you are already embedding IPTC, or do you only want to update an XMP sidecar file if it exists and already contains any IPTC-related data?).
One could write a book on this subject, but by the time it is published, other software may have been updated to catch up with the unfortunate reality of meta data and (those damned proprietary) RAW files. The real unfortunate reality is that different programs have differing levels of support for IPTC/XMP, and that is precisely why we added all those (confusing?) options in Photo Mechanic in order to adjust for different workflows. Just be aware that as you work with different programs, you may need to adjust Photo Mechanic's preferences to accomodate their incompatibilities.
--dennis