According to an interview in Gamesindustry.biz, you were almost playing Portal on Xbox Live.
Valve's Doug Lombardi has said that the company had offered Portal up for inclusion on Xbox LIVE Arcade, but Microsoft declined. Ooh, bad move, Microsoft.
"I'd love to sell Portal on Xbox LIVE," Lombardi told GI at an EA Games Studio Showcase, "[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."
Gamesindustry.biz: Portal was offered to XBLA, but rejected.