Just open System Preferences, and click on Network. Then select "Airport", to show the Airport settings.
If you're using Tiger, you then select your network in the list, and click "Edit" (a small button below the list). In the dialogue box, you check "Show Password". You may have to authenticate to see it, but probably not.
On my 'puter, it shows the Hex equivalent password, which is probably what you want anyway to get your PSP to connect.
I don't remember the exact way to do this in earlier versions of OS X, but I do know that it can be done, and that the information is in the Network panel somewhere. I used to work in a place where we often had visitors from other branches, and I always managed to repress the memory of that particular password, so I've looked it up lots of times...
If you don't find it, start the admin application for your access point (Airport Administrator in /Program/Utilites if you're using Airport), and log in to your router and look it up there.
You could also try using "Keychain Access", as the password is stored there too. In Tiger it is on the keychain "System". You'll have to authenticate to see it in plain text though.