Well, it's not just Safari--Camino, which is Gecko-based (though I don't know if it uses Firefox's popup blocking system) also generates popups on IMDB. Of course, in its case the popup code keeps dropping the main window behind other open ones, which is fantastically annoying.
I'm not going to dig into the code, but I assume it's doing some more advanced trick with the pop-behinds to bypass blockers. They're not foolproof--after all, there is usually an "ok" way to open a popup in direct response to a click. Maybe IMDB is opening the popup in direct response to the click, then subsequently redirecting the original window and trying to bring it to the front.
And actually, ironically, just yesterday I had Safari being overly draconian blocking popups without warning to the point it broke a legitimate site that was trying to open appropriate popups.