The "Maximised window" view in Photoshop is better on Windows: the title bar of the application includes the filename AND the current zoom. This is vital when sharpening for print - Mac only displays the zoom in normal window view when your eye is distracted from the image by all the clutter of the desktop.
Finally, Photoshop CS4 is 64bit on Windows, and 32bit on.
I didn't know that difference existed between Photoshop on Windows and Mac, but that small frustration is worth me not ever having to restart photoshop after it crashes on all of the Windows PCs I've used at home and work.
Having a 64bit version is nice, but even Scott Hansen—of iso50.com fame—who uses Photoshop probably more than anyone, has admitted that it's not really noticeably faster, although it might be if you're working on small to medium files.
http://blog.iso50.com/2008/09/24/cs4-gets-official/