I came across this interesting post by Pamela Redmond Satran (hat tip to The Poetry Foundation's Dispatches: News feature online) about "Maya Angelou's Best Poem Ever"--which was actually written by Satran as a list column for Glamour in 1997.
Satran's experiences, including having the "poem" produced as a book and poster for sale by third parties without credit to her, sound like the kind of worst case scenario nightmare beginning poets envision when they contact me about copyright, publishing rights, having work stolen, etc. If anything, Satran's list proves that rights and original source of publication don't have much impact. If your work isn't safe from being stolen, borrowed, or misidentified when it's been published in something as high profile and commercial as Glamour, you're probably really up the creek when you publish in small journals or post your poems online.
Note that I definitely am NOT saying you shouldn't publish in small journals or post your poems online. My caveat is more about not wasting too much energy worrying about someone stealing your work, a favorite obsession of beginning and unpublished poets (and possibly some experienced ones, too, if they were honest about it). Deal with it if and when it happens, but don't inhibit yourself with too much fretting about it.
There are a lot of issues associated with Satran's incident that are prime discussion material: intellectual property rights (no matter which side of the fence you're on about that), the viral nature of e-mails and the Internet, whether the "fame" of the work should be weighed against the notoriety the original author is cheated out of. I certainly sympathize with Satran--I guess I'm one of those egotists who wants my name associated with something I wrote, if it originally appeared with a byline. But it also steams me that when something gets passed on and on as a "poem," so often it turns out not to be a poem at all--while there are so many great poems, old and new, that do deserve to go viral. (As Satran says regarding her frustrations in trying to regain credit for the piece, "People believing that's Angelou's best poem ever, I figured, is justice enough.")
Pamela Redmond Satran's website has more information about the author and her work--beyond "Maya Angelou's Best Poem Ever."