This is not official yet but the very strong rumours are that Bungie has not been able to get co-op working over Live properly and will drop it now.
I think this means you can kiss the rumours of a 4 player co-op goodbye for good. According Frank O’Conner
We know that people want it and we’re trying to make it happen. I think the biggest problem for us for online co-op is that we have a situation where you can be in a Warthog with five troops, almost a mile away from the other player. That’s a significant challenge. And there’s lots of design things you could do to prevent that from happening, but they would make it not feel like Halo anymore. If we can make it happen in a way that works well, we will – and if it works badly, we won’t.
So lets all pray to the gaming gods that Bungie find a way to make this happen. Co-op is one of the best things about Gears of War and I was really looking forward to it being integrated with Halo 3.
Source: CVG
Last Updated: July 19, 2007
