Bad device-uri "socket://10.0.0.3"!
OK, so I googled around a bit, looked in the readme, and you can do this using the command-line interface too. The program "lpinfo" shows all the visible printers, including "socket://10.0.0.3" which matches the IP address of the printer. I then use "lpadmin" to add the printer. Easy, right? Not so much:
lpadmin: Bad device-uri "socket://10.0.0.3"!
CUPS has a log. Maybe it has useful information on what's going wrong? Not so much:
CUPS-Add-Modify-Printer client-error-not-possible: Bad device-uri "socket://10.0.0.3"!
Two hours of screwing around later, including verifying that I can ping the printer and even telnet to it on all the right ports, I give up and decide to try removing and reinstalling the package. There's too many dependencies for this, so I settle for reconfiguring it. And guess what? CUPS on Debian has a notion of "backend programs" that can be turned off. It is not mentioned in the README. And "socket" was turned off.
So if you have this problem, try:
and ensure that your desired printing interfaces are active.
Filed as Debian bug #599660.