For the sake of making it easier for others to find this thread via a search engine i'll add some keywords :
Mac OSX mouse hover problem
Mac OSX dock magnification not working
Mac OSX mouseover
...

Hi

What fixed this problem (for me) was merely disconnecting the reciever of the mouse (a Logitech MX Laser <some-number>) and connecting it again. Oddly enough it wasn't sufficient to disconnect...