Hi there
as others have said there really ISN't any point in using XP mode especially for running CS4 (or CS5).

EXCEPT in one particular case -- you can still get round it but it's simpler to use a VM for this particular problem.

If you have a Scanner that only works in XP and you want to ACQUIRE images in photoshop directly then its OK to have a separate VM machine installed and run the whole application from that -- its less hassle than scanning from a VM and then transferring the images to your HOST and running Photoshop -- Your choice of course.

For me running a VM is a good idea but I'd use something like vmware player / workstation or vbox. XP mode doesn't cut it for me either in functionality, flexibility or performance and it makes mainitaining the HOST system more fiddly.

Using a separate VM maintains "the integrity" of the host as the VM is just DATA as far as the host is concerned -- you can save and restore VM's with your normal DATA backup software too.

Cheers
jimbo