Nack attributes this move to the decision Apple made last year to stop development of 64-bit support for Carbon, a move which Nack says took Adobe and other third-party developers by surprise. Adobe did make CS3 Intel-compatible, but has retained Carbon as its foundation.
Nack wrote, in a recent blog entry, "At the WWDC show last June, [Adobe and] 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 [instead of] Carbon".
"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."