Okay, so I've been trying to create a simple little project to submit here, but I can't reproduce it in a simple project. I'll keep on trying though.
I did however check all my other projects (running nightly builds in Hudson, so that's easy enough) and there I noticed this problem occurs only on 2 of my projects. I have no clue as to why, since at least 5 of them are similar in structure and had missing versions in their previous iteration... Funny thing is that in those other projects, the enforcer-plugin (or maven itself, I'm still not sure which) doesn't even lookup the latest snapshot from the repository. Like I said, it only does that in 2 projects and both of those then fail.
For my builds, I can surely create a simple workaround (deploy a version without the enforcer, but with the versions for plugins set) and then go on from there, but I think this is a bug that definitely needs fixing!
So, that where I'm at at this point. As soon as I have a reproducible little project, I'll attach it!