A brief refresher from April 2008 on how Mac OS X got to where it is with regard to support for 64-bit apps:
When Leopard was first announced at WWDC 2006 nine months prior, it included full 64-bit support for both Carbon and Cocoa.
64-bit Carbon wasn’t promised to be coming “sometime”, like with, say, resolution independence. It was promised for 10.5.0. And it existed — developer seeds of Leopard up through WWDC 2007 had in-progress 64-bit Carbon libraries, and Adobe engineers were developing against them. Several sources have confirmed to me that Adobe found out that Apple was dropping support for 64-bit Carbon at the same time everyone else outside Apple did: on the first day of WWDC 2007.
If Apple had shipped Leopard with the 64-bit Carbon support promised at WWDC 2006, Photoshop CS4 would run in 64-bit mode on the Mac.