The screen looks like:
Posted my problem to Whirlpool, some suggested that it could be the Expose that is doing the trick. I am still don't know what is the purpose of having the dark border around the screen. I did not change Expose configuration.
After a machine restart, it seems alright where the dark border disappear in all spaces. Weird.
Wonder if this is a problem with Leopard? Or somehow I did something that trigger this dark border to appear?