Q: Why does IE6 + XP SP2 block new windows launched from my onMouseDown handler?
A: We wanted Pop-up Blocker to allow new windows launched as a result of a user initiated action.  We defined 'user initiated action' as a click (or keypress).  A click is a mouseDown followed by a mouseUp.  So you can launch user initiated windows from an onMouseUp or onClick handler, but not a MouseDown handler.