Photoshop Senior Project Manager John Nack posted a piece on his blog called Photoshop, Lightroom, and Adobe's 64-bit roadmap where he divulges that Photoshop (CS4) will be available in 64-bit versions for Windows, but only a 32-bit version for OS X.
It doesn't mean that the Windows version of Photoshop will be twice as fast as the Mac OS X version though. According to Nack, "the speedup due to running in 64-bit mode is around 8-12%."
However, people working with the largest images will benefit from 64-bit processing:
For example, opening a 3.75 gigapixel image on a 4-core machine with 32GB RAM is about 10x faster.
The reason the Windows gets the x64 nod before OS X is because Photoshop is a Carbon application (like Final Cut Pro and iTunes) and Apple decided to drop their 64-bit Carbon efforts at WWDC in June 2007.
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 states that although the Mac x64 revision will take longer to deliver, "our goal is to ship a 64-bit Mac version with Photoshop CS5," which he quickly hedges by saying "but we’ll be better able to assess that goal as we get farther along in the development process."
For more I recommend reading Jon Gruber's detailed analysis on the Photoshop64 situation on Daring Fireball.