The "feature" is that beginning with XP, Windows now automatically moves the cursor to the start button if it is between the button's position and the edge of the screen when you click.
The reason has to do with Fitt's law, which says that a button is easier to hit if it's at the edge of the screen. However, instead of moving the button to the edge of the screen, Microsoft decided to move the cursor to the button***. This also works with the space below the other buttons on the takbar.
Unfortunately, this seems to happen even if the button is hidden by Start Killer.
So basically, if you click between where the start button normally is and the left or bottom edge of the screen is the start menu will still pop up.
At this moment it doesn't bother me so much that Start Killer doesn't prevent this, since I'm mainly using it to disbale the menu from appearing when I press the WIN key, but I am considering replacing the start menu with TLB, and it would probably bother me then.
*** There is a bug in this feature though, that will prevent it from working normally if you use any toolbars that increase the height of the taskbar (even by 1 pixel), such as the language bar or the WMP bar.