See their statement explaining why on PCs get 64bit...
michael
---
"Photoshop CS4 for Microsoft® Windows Vista® runs 64-bit native.
In early testing of 64-bit support in Photoshop for Windows®, overall performance gains ranged from 8% to 12%. Those who work with extremely large files may realize noticeably greater gains in performance, in some cases as dramatic as ten times the previous speed.
The Mac OS 64-bit version is planned for a future release as a result of a change to Apple's platform strategy, which now requires a migration from Apple's Carbon technology to its Cocoa technology in order for an application to run 64-bit native. Before 64-bit support can be added on Mac OS, significant parts of Photoshop and its plug-ins must first be ported from Carbon to Cocoa. The Photoshop team began working on porting Photoshop to Cocoa as soon as Apple announced 64-bit support in Carbon was no longer part of its plans." (
http://www.adobe.com/products/photoshop/photoshop/faq/)