Currently I have a friend using 64bit XP. Most of the programs seems to be working fine with XP. I am going to switch to 64 bit XP too. CS3 taking too much memory bandwidth. After testing my friend's 64bit with 8GB of RAM (CS3 was allocated to about 3.5GB), opening up a 10-15 layers files was an easy task. Everything was a smooth ride using the 64 bit program.
I am not too sure about Vista as I am just guessing some software makers have yet to update to the new Vista 64bit.
Many software run in emulation (Win32) mode in 64bit Vista. Many software works; rather it is the device drivers that may give you trouble.
As for CS3 or later upgrading to 64-bit, Adobe has said that there isn't a needed (yeah right). They forgot that almost all Windows machines in the past 1 year as well as Mac machines are 64bit. For Intel Core2Duo onwards, for AMD for the past 3 years. Furthermore, RAM is so cheap nowadays. With higher MP images and more complex layering, more RAM means faster speed (ask Clown).
It is a pity that Adobe is complacent, knowing that they are the king in image editor. If CS4 is not 64bit, I hope that its sales would tank, just to give them a wakeup call.
ya stupid adobe. They actually doesn't support OSX 10.3 with CS3. I mean CS3 runs ONLY on tiger for MACs..... don't support 64 bit, don't support panther, this is the last version of photoshop I am buying.