The airport seems to have a problem with the PS3's wireless (or the PS3 has a problem with Macs- take your pick). I did find a possible solution, though.
First, connect to the modem directly via ethernet cable and set up a wired connection. At the end of the session write down the DNS addresses the PS3 picks up. Now, once you reconnect the modem as it was before start setting up a wireless connection using the airport and manually set your IP address, subnet mask and DNS addresses. You will likely then have to go into the airport configuration and set up the IP address you gave the PS3 as a reserved IP (maybe 10.1.2.210 will work).
Also, if you have encryption in place you will have to di around the airport's menus to find the equivalent WEP key.