Some other interesting points not mentioned yet (from the Apple site)...
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Multicore apps in Leopard.
Apple engineers have updated several applications in Leopard including Mail, Address Book, and Font Utility to be fully multicore ready. Each of these apps breaks up processor-intensive actions into a series of more manageable steps that execute one by one on single-CPU computers and in parallel on newer, multicore systems. Cocoa uses that same technology to speed up Spotlight searches and Dictionary lookups."
...more....
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Smooth operator.
How did Apple engineers pull this off? By using NSOperation, a breakthrough new API that optimizes applications for the world of multicore processing. Independent chunks of computation (operations) are added to an NSOperationQueue, which dynamically determines how many operations to run in parallel based on the current architectures. So theres no need to hand-code the complexities of threading and locking. You simply describe the operations in a program along with their dependencies. Cocoa takes care of the rest."