Well, there are also agreements about a Doom Movie.
It's interesting, but most of these projects are finally dismissed, since the success is still unsure. However, it's very possible (although pathetic, imo </sarcasm>) that Halo makes it to the big screen. All the money it has brought Microsoft is enough to make a Holywood superproduction by themselves.
MGS may have a nicer way into the big screen, because its story is more cinematographic, but it's not sure it will success on screens...
We talk about money, not about art anymore.