A couple of days ago, I was in a conference call with Adobe. John Nack, the product manager for Photoshop, wanted to tell us about what 64-bit computers mean for Photoshop. The bottom line is that while the Windows version of Photoshop will be 64-bit, the Mac version will remain 32-bit. Here's the important bits:

This is not because Adobe hates the Mac as a platform; it's to do with changes Apple made to the tools companies use to develop applications.

Most people shouldn't care – you only see the advantage of 64-bit apps if you need to access huge – more than 4GB – amounts of RAM; in the case of Photoshop, this means working on massive images. The huge majority of folks are likely to see only very minor speed increases.

Nack's post on the subject is a great primer if you want to know more about the switch from Carbon to Cocoa, and the ever-excellent John Gruber has his own post-match analysis here.

And yes, the Mac will get a 64-bit version of Photoshop; if all goes to plan, it should be included as part of CS5.