Expocity is a hack that was proposed yesterday for inclusion into the Metacity (GNOME window manager) CVS tree. It probably won't surface for a while in any of the releases, but if you're using GNOME 2.5.x out of CVS you may get a chance to play with it soon.
I'm pretty sure it's not nearly as pretty or lickable as the real Expos�, but it does thumbnailing and automagically shows you all your windows. This combined with the new XServer stuff might bring the Apple eye candy a bit closer for Linux users (Windows folks will still have to wait until [2006 or whenever] Longhorn is released.