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).
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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]
DISCUSSION
I don't see the need for scroll bars at all especially with scroll-wheels and multitouch. Just make it like the iPhone/Touch where it appears and then disappears when necessary.
Also, the translucent black-glass reminds me a bit of vista.