Previewed at WWDC, launching in the fall.
Mac OS X 10.4 (Tiger) Screenshots... Real. [Updated]
While sources have been unable to directly confirm the validity of the images posted, we have been able to get reliable confirmation that many of the concepts -- including the Safari RSS implementation as well as the Konfabulator-like widget plug in system -- should find their way into Mac OS X 10.4.
Mac OS X 10.4 (Tiger) will be previewed this next week at Apple's World Wide Developer's Conference.
Update: ThinkSecret reports that one reliable source confirms the screenshots are genuine.
Update 2: Images removed at request of Apple Legal.