Adobe is working on a new version of its Creative Suite, that includes Photoshop, Illustrator, Dreamweaver, and other applications for print and webdesign - Adobe CS4. The new version of Photoshop (Windows version) will support 64-bit memory addressing.
Why it will support just Windows (Vista)?
Well, Photoshop for Mac OS is written using older APIs that don’t allow access to all the latest Mac OS X features. To make a new version it will mean a total rewrite of the code and it seems Adobe doesn’t have time for that.