Just like the answer-posts from Aug 2009 wrote down about the UPnP - turn it off on your router admin and the PS3 connects like a champ to your friends' invites. I had the same problem for three months (after installing new G router with higher security and encyption; WEP sucks) and tried many "fixes" found online. None worked until this one.
Just go to your router's admin settings and turn off the UPnP and refresh your PS3's internet connection settings. I called my friend to get onto COD4 and CODWaW, and we connected to each others invites with no problems.