AppleInsider's sources tell them that Snow Leopard, previously thought to only get under the hood upgrades, will get a new UI codenamed "Marble".

It will supposedly feature flat dark scroll bars like those in iTunes (below).

AppleInsider also says the new OS will include a minimalist window design, as seen above, with translucent black-glass headers. I am a fan of these minor UI upgrades that somehow imply freshness. [Appleinsider]