I keep CS1 and 2 on a separate HDD, and Keep CS3 and 4 on the laptop and desktop, and keep every scratch disk for each system on a third HDD connected for that explicit reason. I have never had any problems…. on my mac. On windows, I actually watched it shoot its self in the face… 12 times. I counted. If I ever have to go back to windows, dear god, no, I would install everything but Vista, the 3,000 extra processes Vista installs and needs to run, and CS4.
But ya, I have not had any problems, unless it is with a file. For the longest time Photoshop CS3 wanted to open all my old PDF’s, until I figured out how to get mac to force things to open with an certain App.