http://community.callofduty.com/thread/100486208
Relevant Section for reference:
= Sign into Xbox Live at the Dashboard
= Launch the game (installed on HDD, as an aside)
= Launch Multiplayer
= Select Xbox Live (at this point I can see a number of players online, so I'm obviously connected to something.
= Select Player Match
= Optional step - if you select Find Match, all of the options (Core, Barebones, Hardcore, First Strike) are greyed out and not selectable
= After about 30 seconds, though it varies, I receive 'You must have an active network connection to play Xbox Live or System Link matches'
= Once this has happened once, the message will appear at random intervals
As a point of comparison, I can find a friend/recent gamer from the list with a current session and choose 'Join Session in Progress' and I can connect and play no problems.
Any thoughts?