It would take an awful lot of adaptations to make it work properly as a co-op title I imagine. In Borderlands by comparison, you have smaller areas which all players are forced into once one player initiates the transition, and you also have game dynamics which don't activate until all players are present. With Fallout the entire wasteland will be open, so you could very well end up miles and miles away from your co-op partner. You could also have a situation where one player misses out on some game-play because your co-op partner stumbled across something on their way to you and inadvertently completed a mission...
Games like Borderlands are semi-path driven, semi free roam, with forced transitions for all players. So it works a-lot better than it would for Elder Scrolls/Fallout.
Personally I don't really want co-op.