Upload & Sell: Off
Over at the Adobe forums, the consensus seems to be no real trouble per se with Mountain Lion and versions of Photoshop (CS4) even older than 5 - it's the plug-ins (including some of the older Adobe plugins) you have to worry about.
Besides looking to the websites, you could do a preliminary scan for old PC code: go to the Apple Menu, click on 'About this Mac', and then hit the 'More Info' button. From there click on 'Applications' and look in the right-hand 'Kind' column for anything that shows 'PowerPC' rather than 'Intel' or 'Universal'. PowerPC code won't work in 10.8 - any Power PC apps or plugins will have to be updated. One casualty is the old Photoshop plugin, 'Contact Sheet'. It's PowerPC, and If you've been dragging it forward into newer versions of Photoshop, its career comes to an end with Mountain Lion.