I'm reasonably sure that MC supports IPTC for reading only at this time, and that any tags that are saved out to the image files are saved in "proprietary" tags. I'm not positive on this, but it does not seem to interchange information with my other applications.
I would, of course, really prefer that it support both reading and writing of both XMP and IPTC tags. Any media management app that really wants to be able to handle images in a professional environment needs, at a minimum, full IPTC read/write support (and XMP is all-but-required and next year at this time it really will be).