Yep, confirmed here, bug is only with vertical switchbar on left side of window, in both XP and win2k skins. For all other switchbar positions, the tooltip moves off the button, for the left switchbar, the tooltip keeps appearing in the left bottom corner of the button, even if the mouse is there...
Ofcourse, you need a button that actually generates a tooltip, so use the buttons for status window or long channel names.