Hi Steve,
I attached my 1600n directly via USB this morning (in an effort to get my Mac to recognize it as a TWAIN device), and it was recognized in the printer control panel as 'Laser MFP 1600n' (connection via USB). From there I clicked on Printer Setup, and chose the Xerox WC PE120 Series PS as the printer model.
I did notice that in the printer browser (by clicking the + button) that there is a Xerox 1600n listed with a connection of 'Bonjour'. When I tried that one I got the same error you did.
Mac Mini 1.83GHz Mac OS X (10.4.9)
An update on the scanning: still no luck. Even directly connected to the Mac via USB it won't find it as a TWAIN device.
I did, however, find a handy guide to cleaning the scanner glass. You may have heard of the wide spread problem with the glass getting a hazy film on the under side. This may help:
http://www.prism.net/user/rtadams/Dell1600nFix/How%20to%20fix%20a%20foggy%20glas s%20in%20a%20Dell%201600n%20laser%20printer.htm
Cheers,
Tucker.
Mac Mini 1.83GHz Mac OS X (10.4.9)