I'm not sure if this will help anyone, but here's what I did to open my NAT on PS3. Forwarding ports alone didn't do the trick.
This worked for both my Actiontec M1000 and Q1000 routers.
1. Set up a static IP for my ps3
2. Set up a DMZ through my router
3. DISABLED Upnp on the ps3 only(not sure why this worked, but it's moderate with it enabled)
4. Did the MW2 trick
NAT has been open ever since(months). If my router is unplugged or reset, I have to do it all over again, but it's no big deal. Hope this helps. I'm not sure how common these routers are, but they're kind of a PITA.