OS X may indeed touch the file (for example, it may read the file's name and add the file to the Spotlight index), but that doesn't actually change the file.
I am skeptical of Adobe's claims, since earlier versions of Photoshop don't have this problem. There are actually several different dates and times stored with a file on OS X: the date and time the file was created, the date and time the file was modified on disk, and the date and time the file was last accessed. I think, based on what I've observed, that in the past, Photoshop looked at the date and time the file was modified, which is what it should do; it appears that CS5 looks at the date and time the file was accessed, which is the wrong thing to do (as the file may be accessed by, for example, indexing it or placing it in another program--but that does not actually modify it).
