Windows gets it and Macs don't. The former will have a 64-bit version of Photoshop CS4 and the later won't.
The 64 bit version of Photoshop offers users an easier way to use more than 4GB of memory when using the program.
The functionality of the program also depends on the kind of chip you have. The 64-bit Intel and AMDx86 have more registers (areas to store data) that effects the speed at which the program operates. Photoshop CS4 will be able to operate at 64 bits with these chips.
It really isn't a big deal which version of Photoshop--64-bit or 32-bit-- you have, not yet anyway. None of the 64-bit designs are not used in the real world because the files haven't grown that big.
But they will, and soon.
Over the next two years when 8GB of memory will be commonplace, the differences in speed between the 64-bit and 32-bit versions of Photoshop CS4 will become substantial. That's because as images are increasing in clarity and size, and you need more memory to handle the files.