[Stuart]

I don't have a PS3 to check this with, but here is how to set up the Mac for the networking side. Assuming you have OSX 10.5 (leopard)

1. Open System Preferences (click the Apple logo, then System Preferences)
2. Click "Networking"
3. Click "Airport"
4. Now, click the box next to "Network Name", and select "Create Network"
5. Enter a name for the network. Whether you want a password or not is up to you (I would, however, recommend you do).
6. If you tick the password option, enter a password and select 40bit WEP or 128 bit WEP (sadly, OSX doesn't appear to support WPA on this kind of connection). Enter the password again in the Verify box.
7. Click OK

As for setting up the PS3, someone else will have to answer that.

I would, however, like to echo the above. Using a wireless router for this sort of thing is usually a lot better.