Photoshop offers huge speed advantages for those who typically work with large images on systems with RAM of about 32GB and up. Even normal users will possibly see a performance increase of 8% to 12% when using the 64-bit version of Adobe's software compared to 32-bit.
Regrettably this means Mac OS X users will not be able to take advantage of the performance gains in CS4; those who do manipulate massive images on their Mac's may need to use Windows until CS5 is released further down the road.
A Note: Ironically, when Photoshop was initially released back in 1988, the software was available for Macintosh only. The product didn't even ship for Microsoft's platform until 1992, when Adobe ported Photoshop 2.0 to Windows.