I found out how to access the multiplayer mode without xbox live (but you need to be able to access it) for the achievement "get a life..."!
First you need an unpatched game. If you do not have one go to system settings then memory and press Y for device options. Now you see the option clear system cache. Press this and then yes. Don't worry only the patches for your games are deleted.
Now play UT3. After you started the game you will see the good' ol update question. Say No and go to multiplayer. Now you are able to play multiplayer mode without going online!
After you got your 200 kills deactivate xbox live (e.g. pull out your LAN cable)
Go to system settings go into future one day. Now start UT3 again and plug in you cable (or whatever you're using to go online) and to the same as above until you have lost your life.
Because you have access to xbox live but aren't logged in because of the update message you can access the multiplayer mode and still be able to keep the date!