What they should do with Fallout 3 is have a saved co-op game of some sort.
You and the one friend will specifically be the only people to use that one save, and how it would work is:
You both start off at the Vault etc...with your friend being marked on your map and maybe a vault-boy head on the compass marking your friend's location. It would be one of those "help your friend or not" kind of deals. When you want to save, the game would pause and ask your friend if he wishes to save as well. If he accepts, the game is saved. If he doesn't want to save, vice-versa. If either of you die, the most recent save would be loaded etc...
Should the friend leave, the game saves and it loads your singleplayer game. The MP save would be there whenever you choose to do another co-op session.
....In my dreams.
