OK that helps a little. It might be Lion that snagged you. Custom sizes are actually a function of the OS, and neither photoshop or the printer driver really should affect anything.
The file you are looking for should be in the users library folder, path is ~/Library/Preferences/com.apple.print.custompapers.plist
If you did a clean Lion install you may not be able to move that file over directly to the Lion install from a Snow Leopard backup, something about regular XML and binary. Here is a link that talks about the issue and how to convert the SL version to the Lion version.
http://hints.macworld.com/article.php?story=20120502125306691
hope that helps
(I didn't have issues with this because I decided to do a clean install with Lion and my custom paper list was so junked up I just rebuilt it as I went a long.)
Thanks again Wayne. For some reason I was searching for a file with some sort of esoteric name and extension. Thanks to you I have now found the one I've been looking for, and I have seen this file before, just couldn't remember where. Mine was a clean Lion install as well, so no custom sizes obviously. As I had mentioned, it was essentially an upgrade, but with a new SN. I originally thought I'd have to have CS3 installed on the Mac Pro so that CS5 could pick up the CS3 SN, but of course that was not the case. Another issue for me is the fact that I've just upgraded from a G5 that I've been using very successfully for many years, to a Mac Pro. Therefore, a number of things to contend with. It has however been an interesting experience, lots of time spent on forums and other research media before making the move. Obviously I'm finding the MP a pleasure to work with, but of course the new OS is another challenge to some extent. What I'm planning to do is copy the .plist file on the G5 and paste it into the users library on the MP. Restart and see if it works. Of course before I do that I'll read the article Macworld you linked. I've been running Leopard on the G5, but I imagine the conversion process might work. The way I see it, no harm in trying. And while I was finding the Custom Papers file I also noticed another file I've bee looking for. That being the custom presets list. I'll try to make this one work as well. Yes, I could and probably will have to rebuild my paper sizes and presets, but I'll try what 'might' be the less time consuming approach first.
I'll let you know how this works, or doesn't, and thanks again Wayne.
Gary