I made it about 3 days before I was disgusted with the Blue glow, border, shadow or whatever you want to call it now found in Snow Leopard's (SL) Expose. Was really wishing they kept the look of the old Expose. I just can't believe they would pick such a horrific color with all the cool transparencies and growl looks found in Snow Leopard. The dock even has the very cool transparency. And there is no where in the System Preferences to remove or change it. Luckly I found a solution in the forums at MacThemes by dementian27.
So you can take that blue (puke).
To a very Mac looking Transparent Glow.
This can be done very easily by just replacing 2 files.
Download dementian27's transparent files Dementian27_expose_glow_replacements.zip
They are located at: System/Library/CoreServices/Dock.app
(right click: show package contents)
The images to be changed are: expose-window-selection-big.png for normal windows and: expose-window-selection-small.png for minimized windows.
Just replace: expose-window-selection-big.png and expose-window-selection-small.png with the ones in the zip.
Next log out and log in. Now the Exposé Glow should match the coolness of everything else about your Mac.