Adobe Photoshop CS4 to be 64-Bit for Windows, 32-Bit for Mac
First off, Nack admits that the 64-bit version of Photoshop will see modest speed increases (8-12%) but the biggest advantage will be for those using massive images (a 3.375 gigapixel image is given as an example). With these massive sizes and with enough RAM (32GB given as example), you can see substantial (10x) improvements.
Adobe expects that Mac users will have to wait until CS5 before getting full 64-bit support. The reason for this delay is due to Apple's abrupt dropping of Carbon 64-bit support:
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.
Nack reports that they started working on the transition immediately after the announcement but due to the scope of the transition are unable to deliver it by CS4. He also tries to dispel the notion that Cocoa alone will produce a higher quality product than Carbon:
Most Mac users don't know Cocoa from Ovaltine, and nor should they: it's just an implementation detail, not a measure of quality. I think Brent Simmons, creator of wonderful Cocoa apps like NetNewsWire, put it most elegantly: "Finder + Cocoa = Finder." That is, rewriting one's app in Cocoa doesn't somehow automatically improve its speed, usability, or feature set.