I'm on a 3.06 GHz iMac running OS-X 10.6.1
Opening the G-Force Toolbar with iTunes 9 running, the "Target G-Force Visualizer" drop-down menu is blank and everthing else is grayed out.
edit: I should have mentioned that this is new, and the Toolbar used to work normally on this machine with 10.5.8 and with the G-Force version before 3.9.2.