Someone on iLounge did just that, and Contour's response was that it would work, but a new Showcase was being worked on at the moment.
Somebody else had just taken the plunge with mixed results. The supplied foam inserts for smaller iPods didn't help the fit, and there is a large area around the click wheel which is prone to scratching - mainly down to the fact the new iPod has a smaller click wheel.
I think you'll be better getting a temporary case and getting a 5G specific Showcase when they are released. I'm a huge fan of the Showcase, and won't be getting a 5G until they are available - my 4G went straight into one and never came out again until the day it was sold - scratch free!