At the WWDC show last June, however, Adobe & other developers learned that Apple had decided to stop their Carbon 64 efforts. This means that 64-bit Mac apps need to be written to use Cocoa (as Lightroom is) instead of Carbon. This means that we'll need to rewrite large parts of Photoshop and its plug-ins (potentially affecting over a million lines of code) to move it from Carbon to Cocoa.
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John Nack on Adobe: Photoshop, Lightroom, and Adobe's 64-bit roadmap
So… Windows will get a 64-bit version of Photoshop in CS4, Mac will not. Bummer. Real big bummer.