An old post, but deservs a bump with this addendum:
I see someone people complaining that they put their PS3s in the DMZ, and still have issues after port forwarding.
Because it is redundant and you are confusing the router. Having a device in the DMZ means that all the ports to that device are open, so port forwarding is NOT required.
If you are having issues with your NAT type/Joining parties etc:
1 - Assign a Static IP to the PS3 and port forward (look up the ports) - safe but can be problematic if not all the required ports are open
2 - Put the PS3 in the DMZ (procedures on how to do this exist) - safe for a PS3, but not you PC (as an example)
3 - Enable uPnP on your router - debatable how safe this is (see above), but the easiest setup