A few of my geekiest and most nit-pickiest brain cells just exploded. After reading about Safari's odd tab dragging behavior, on two different Web sites on the same day, I am not sure I can take much more.
Both Pierre Igot and John Gruber cover this topic in great detail; the fact that the first user interaction with Safari's tabs while dragging ultimately determine their ability to either reorder the tab and/or open a new window/tab. But that's generalizing it far too much.
If you're an anal Safari nut, like me, I suggest you read both of their articles.
Secondary, source: Safari's Tab Dragging Modes.