I've had sporadic problems on my new laptop and my old mac mini with the mouse hover events not being fired which causes problems for exposé and the dock and hoverovers in web pages. Until now I'd never connected the fact that I was using the same wireless mouse on both computers - it seems there's a problem with Logitech mice that causes this to happen. I've finally found a fix - unplug the mouse hub, reset the mouse and plug back in.