This is basically a note for myself as I have come up against this problem three times now and each time had to work it out! (which I put down the bodies natural reaction of forgetting pain!)
If you experience the same as me you will find that wireless connections are fine and MSN connects no problem but with wired connections, wether direct or through another switch wont and throw the error 81000306, troubleshooting with Messenger all passes and specific connection tests (like TCP and HTTP from connections->Advanced settings) show all is fine, the connection tab will even say "You are directly connected to the .NET Messenger Service", but alas still wont connect.
Well, the problem is in the router firmware, the version that has problems is the latest version, 3.01.31. Revert back to the next version, 3.01.25 and the problem goes away!
Happy Days :)