Right after announcing the release of 64-bit support
in the Lightroom 2 beta, Photoshop Senior Project Manager John Nack dropped a little bombshell
on his blog, announcing that the next version of Photoshop (CS4) will be available in both 32- and 64-bit versions for Windows, but only a 32-bit version for OS X. The reason: Photoshop on OS X is written with the Carbon API. Last June Apple decided to kill 64-bit Carbon, forcing all future 64-bit application development on the Mac to Cocoa. Adobe had originally planned to ship a 64-bit CS4 as a Carbon app and port to Cocoa for CS5, but now the 64-bit version will have to wait for the CS5 Cocoa build.
The (relative) good news is that this will primarily affect users working on "very large files on a suitably equipped machine." The average speed bump "due to running in 64-bit mode is around 8-12%" when not "using a large data set." The bad news is that it is precisely those professional users who buy Mac Pros maxed out on RAM that will likely get penalized. It'll be a sad day when the Photoshop jockeys have to run in Boot Camp to get the most out of their Mac Pros.
[via Daring Fireball