Mac OS X users will have to wait because of an incompatibility caused by a change made last year by Apple to programming underpinnings, John Nack, Adobe's product manager for Photoshop, said on his blog.

At the WWDC show last June, however, Adobe & 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 (as Lightroom is) instead of Carbon” Nack said.

In consequence the Adobe team 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.

"We're not going to ship 64-bit native for Mac with CS4," Nack said. "We respect Apple's need to balance their resources and make decisions right for that platform. But it does have an impact on developers."

It appears that the 64-bit version of Photoshop will bring a performance increase of about 8 to 12 percent, compared to the old 32-bit version. "The real strength of 64-bit's is letting you address a large amount of memory. You'll see the biggest difference when working with large images or moving around a lot of data," Nack explained. To take advantage of 64-bit you need "more than 4GB Ram allocated to Photoshop," he added.

Although a shipping date for the new version of Creative Suite hasn’t yet been revealed, what matters is that it is coming. 

„64-bit computing is an important part of the Photoshop and Lightroom story going forward, but it's not a magic bullet and we're not going to oversell it as one. We're delighted to be offering a 64-bit-native Lightroom on both Mac and Windows now, and to deliver a 64-bit-native Photoshop on Windows as part of the next release. As for Mac x64, we'll continue working closely with Apple (just as we've been doing) to make the transition as quickly and efficiently as possible” expalined Nash.