My method, which REALLY works in 1-2 attempts:
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1) Install all file supplyed
2) Open "Proxy Server" on your PC
3) Enter your PC IP address in the box asking for IP address (Start>Run>cmd>ipconfig>ip address)
4) Now go to the PS3 internet conection setup. Select "Wired" or "Wireless", then custom. Keep all settings automatic apart from Proxy. Choose "Use Proxy Server" and for the "Address" put your PCs IP adress. Then finish/save and DONT test connection
5) Make sure you are signed out
6) Open "CommView" on your PC and click on the large "Start"-Button on "Proxy Server"
7) Go to your PS3. Game Data Utility>Call Of Duty 4: Modern Warfare>Triangle>"Delete"

Press the "Play"-Button on "CommView" and then Sign In on the PlayStation 3. Once signed in, press the "Stop"-Button on "CommView"
9) Start "Call Of Duty 4: Modern Warfare", skip all "New update is avalible" messages. Go to "Multiplayer" and get ready to go online. Go to "Play Online". but DONT sign in yet! Go into the next step
10) Press the "Play"-Button on "CommView". Now click on "Pakets". Then right click in the list of pakets. Choose "Send>All Pakets". Another screen will pop up, cheack the little circle next to continues and click "send"
11) Now atempt to sign in and bypass all the time. This will take a FEW attemps. Keep signing in over and over until you are online