Do this if you can on one of your 10.8 Macs that should be pulling updates from your SUS-
defaults read /Library/Preferences/com.apple.SoftwareUpdate
Take a look at the first CatalogURL line, if its there. It may read as the server address of your SUS, but see if there is a trailing space at the end of the URL. In other words, it should read something like:
CatalogURL = "http://192.168.1.200:8088/index.sucatalog";
but it may in fact read as:
CatalogURL = "http://192.168.1.200:8088/index.sucatalog ";
Note the space at the end right before the closing quote mark.
If you see that, then that's the problem. In some cases, the JSS is adding a trailing space at the end of the URL, which apparently causes Mountain Lion to default back to Apple's servers. Why it doesn't just fail since the address is invalid is unknown to me. Apple trying to be 'customer oriented' I guess. : /