8.04 on desktop switch gray bar in top left corner

[sfkrystal]

Just installed Ubuntu 8.04 and turned on the ATI accelerated graphics driver so I could use the normal visual effects. As far as I can tell, this is the only problem:

Whenever I switch desktops, either by using ctrl-alt-arrow or just by clicking on a different one in the panel, it puts a gray line in the top left corner that goes away when I click on the desktop.

btw, I don't use a top panel.

[sfkrystal]

here's a screenshot: http://bluefishe.com/img/ubu/ubu.jpg

and when I put the bottom panel on the side, it changes color: http://bluefishe.com/img/ubu/ubu1.jpg