Kirk,
OK...I tried what you suggested, and now feel that it may be a card issue, or OSX not allowing the eject of the card to have been the problem. I tried this three separate times, with two different cards, and for the most part they are all now injesting fine and applying my IPTC pad to the images. One one card, there are 12 files that are reporting the 'unsupported' message, but in looking at those files on the card via the finder, they are all zero file size, so that's either a card issue or a camera problem. So, I'm gonna do more testing on that specific card and see if perhaps there's a problem there. It's been used many, many times, so it could be the issue.
The second card that the problem happened with yesterday worked fine today when doing the testing, and all of those files are fine and have the proper IPTC info applied when testing them just now.
Overall, I'm not sure what the issue was yesterday, but am now leaning toward a problem with the card, and NOT PhotoMechanic. I certainly appreciate the quick response to this issue - as always, you guys ROCK!!