As you may or may not be aware, Apple Mac OSX actually runs CUPS as its printing system, its just pretty good at hiding this. One of the quirks of the OSX CUPS implementation is that its configured by default to ignore other CUPS servers on the network, even if you tell it to look for shared printers on other servers. To rectify this we need to go into the CUPS management UI.
Open your web browser and browse to http://localhost:631, you should be presented with the CUPS UI, cool huh! Click on 'Administration' to get to the juicy bits.
Click on 'Advanced' in the server settings section to show the protocol options, select CUPS.
Click on the 'Change Settings' button to save your configuration and restart CUPS, you will be asked for your administration credentials.