Do we know this for sure? i thought the interface graphics were optionally bundled inside the app? AFAIK apps can still look brushed metal, its just that apple has unified the software that comes with leopard.
I know that you could use safari as an example of an app thats brushed in tiger and not in Leopard but I'm pretty sure that Leopard Safari is either a unique build or it detects Leopard. Otherwise we would see webclips in Tigers Safari.