Although Portal (Xbox 360, PS3, PC) was released along with a bunch of other title in The Orange Box, Valve once had the idea to release the game via Xbox Live. Unfortunately, Microsoft turned down the idea.

Doug Lombardi, marketing director at Valve, explained that there were technical aspects that wouldn't let the game be distributed via XBL:

We'd love to do that. Right now it's something we'd love to do. I'd love to sell Portal on Xbox Live [but] the platform holders aren't doing that right now. There's a size limit and all kinds of other things.

We've asked them, we said we were open to it. So it's a decision for the platform holder and how they want to make the games available and how much bandwidth they want to [allow].

It always happens once it's been proven and I think it's been proven now on Steam, so I'm sure it'll migrate back to the consoles just like everything else does.

Well who knows? Maybe Portal's sequel will be the one to hit XBL. Last year, Microsoft increased the size limit of Xbox Live Arcade games from 50 MB to 150 MB. Maybe they'll stretch it even further.