Hi @EAMann and others:
I've contemplated this question a lot before you raised it and I think this answer is not only is it not something you can really do anything about, I don't think it is something we want to discourage.
Writing code is just one piece of execution; the fact they put up a plugin where you didn't shows they are executing more effectively (note, I struggle with execution all the time, so I'm keenly aware of my own shortcomings in this area.) Remember they also have to document and support a plugin which you do not have to do just by writing the code. So even though I know it feels bad in the gut, I think it's a good thing that it happens.
(As an aside, I hate going to the websites of people who are asking really n00b questions here and on LinkedIn's WordPress group only to find their site boasting about what experts they are. But I digress...)
Another way to look at it is, how are you harmed? If they published something and attributed you and then it failed you might be served as a defendant in a lawsuit (stranger things have happened.) But if they didn't mention you, how can it affect you? Unless of course they competed with you for a project and they won while you lost. But that's so unlikely, and if you know it then maybe you can mention it to the lost client? (dunno if I would though.) And if they get business using your stuff for which you were not in competition, be happy for them and glad you could help! If they didn't attribute you, karma's a bitch and it'll likely come around to bite them.
What's more, I think you'll find that you've learned a tremendous amount from others writings and I'll bet you've written code based on the writings of others. You learn from them and then you internalize the knowledge and you often forget where you learned about it. I know I learn a ton from others but rarely remember where I learned if it's been more than a few months since learning.
The real issue here is if we started to police everyone's usage of code here then we'd all have to be careful of using others code when we were not even realizing we were. I really don't want to have to start worrying about whether I can use a certain technique or not based on where I first read about it.
Better that we all stand on the shoulders of giants and "rise the tide that floats all boats." If you want to claim a spot for your own that is based on your code, such as the publishing of a specific plugin, then by all means do it first and/or do it best. Beyond that maybe you can also learn to take joy in the fact that you've become a very effective catalyst?
Or you can stop publishing code here and then nobody can steal it. But we definitely don't want to see that happen because your contributions are great and loosing you wouldn't really benefit anyone, least of all you. :)