I received a save-the-date email from someone about her own baby shower. It was someone I had just met and barely knew (I do know her parents, however). I thought it was weird that she'd invite me anyway and even weirder that she, and not the hostess of the baby shower, sent the save-the-date email. So, when it turned out that I was not actually invited to the shower, I happily let it go.

That said, I've read a number of things about the topic which suggest that while a save-the-date card is not the same as a formal invitation, you should not "uninvite" someone who received a save-the-date care by simply not sending them an invitation. That's in horribly bad form. I think SK's suggestion to casually mention it is the way to go. They may get flustered or have to admit their faux pas, but so be it. If there's a reason they've had to uninvite you, then they should have the courtesy to at least explain it--and apologize profusely for it.