I know there are some posts dealing with this, but I'm hoping this will be the "be all end all" on this matter. If it fits the bill, could this be sticky'd?
I had been dealing with some serious online / activision / PSN / IW BS--unable to join friends games, unable to accept invites, friends list disappearing, lag between XMB, messaging, class selection etc-- and after some help from Syn and the links I'm providing below, things seem to be much much better.
Getting a Static IP for your PS3:
PortForward.com - Free Help Setting up Your Router or Firewall
How to get a Type 2 (Open) NAT on your PS3
The ULTIMATE ultimate guide to fixing your NAT - PlayStation.com Forums
PortForward.com - Port Forwarding Guides Listed by Manufacturer and Model
---> Find your router in the selection.
Click the "skip this advertisement" link in the TOP RIGHT.
--->IN the "P" section of the next page, got to the "Call of Duty Modern Warfare 2" link. Click.
--->Enter Static IP from what was done earlier
Follow remaining steps according to portforward.com's instruction.
Post 'em here.
Hopefully someone can help out.