I had absolutely wanted to know--as we all have--whether rumors of directionally opposite toilet flushes in the northern and southern hemispheres had been greatly exaggerated. And so I despaired when I arrived here and realized that I hadn't taken note of the flush-direction of my trusty American toilet before I left (I have a horrible memory for small details like that). So I owe a big debt of gratitude to Matt Yglesias for asking me about this via instant message.
My understanding had always been that the rumors were true and that the Coriolis force was responsible for the phenomenon. But then I went to college where I learned that the Coriolis effect is negligible over small distances (like, say, the diameter of a toilet bowl).
So, having been disabused of Coriolis disinformation for a few years now, Matt and I conducted a (wasteful) experiment. It turns out that when you fill up and then drain my kitchen sink here in Buenos Aires it drains clockwise. AND, perhaps evidince of swirl-rumor confirmation, Matt's toilet flushes counter clockwise. But nothing's ever that simple. Because my toilet down here flushes counter clockwise as well.
I have no idea how regular the hemisphere-wide toilet-flush trends are, and if there is a physical explanation, I have no idea what it is. My guess would be that swirl direction is purely a function of how the relevant pipes are structured. But I'll leave that to the blogosphere (which I know to be a trustworthy and mature resource) to decide.