Once upon a time there were two companies that didn't know jack, and here's my take on the whole situation:
Valve: "HL2 won't be out on Xbox!"
Microsoft: "What? It won't be out on Xbox?"
Valve: "Uhm, It might be out on Xbox I guess.."
Microsoft: "So it will be out for Xbox?"
Valve: "Actually it depends, we're not sure"
Microsoft: "We have absolutely no idea"
Valve: "So now you don't want HL2 for Xbox?"
Microsoft: "We never said that, you're misunderstanding"
Valve: "You're calling us stupid now?"
Microsoft: "Now you're just giving us mixed messages!"
Valve: "Ok, I'll tell you what, we'll just go ahead and make HL2 for Xbox, and you can decide if you want it or not, ok?"
Microsoft: "Does that mean HL2 is coming for Xbox then? No more misunderstandings and what not...!?"
Valve: "SHOW ME THE MONEEEEY!"