It's all or nothing. Or something later. Halo creator Bungie is having probs getting Halo 3's co-op mode up and running. Says Bungie's Frankie (not pictured):

We're not dumb. 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.

That being said, if they cannot get it to work, Bungie would try to make co-op available as a download after the game launches on September 25th. Here's hoping they figure it out in time. Until then, relax.
Co-op Probs [VGB]

Contact information for this author is not available.