I've spent some time recently with DTrace, a performance analysis and debugging framework that ships on every Mac running Leopard. It's an extremely powerful framework that many Cocoa programmers disregard due to its apparent complexity.
I found that DTrace is actually reasonably easy to use, and have collected my experiences with it in a two-part article. The good people at MacResearch have published the two parts here and here. I invite you to check them out and hopefully learn a new way to tune your Mac or iPhone applications.