FYI
Xbox games are not actually coming. I don't know why anyone would take that idea seriously to any degree, but apparently some people have. Just wanted to quell that this was a joke I started.
I really like starting jokes, or making jokes, and I saw the website being down as a great opportunity to start a seriously ridiculous rumor. As cool as it might be to see actual original Xbox games on this service, I do believe that Microsoft would really take issue with that idea, and I think they would come out on GFWL instead of GOG.
I know that many of you may be disappointed or find it frustrating that I would start a rumor, and think it kind of malicious, but you have to understand that I really love making jokes.


tl;dr
Xbox games like Halo and Halo 2 are definitely coming to GOG.
Official Statement from GOG

Alright, you got me there - who's STILL taking you seriously?

Can't figure it out, not by any stretch of my imagination.

I don't think they're likely to post.

Is there a good Xbox emulator yet? I doubt it, but I would love it if there were. There were quite a few Xbox games that never got PC ports which I wouldn't mind playing, like say 007 Everything or Nothing.

Plenty of Xbox games can't even run on an Xbox 360. It's very sad. :(