Well...crap. I was wrong. If you listened to the latest Co-Opticast I managed to open my big mouth and pretty much guarantee Serious Sam HD on the Xbox would be released this week or next week. According to a tweet from the game's developer, Devolver Digital, it's not hitting till next year now.
Serious Sam HD: The First Encounter on XBLA got the 'ol bumperooski to January. Get you Microsoft moon dollars ready...
I gave in and snagged the Steam version this week. Let me just say 16 player co-op is completely insane, and the game looks and plays beautifully. We'll have a full review soon.
The Xbox Live Arcade version is currently planned to be capped at 4 players in co-op.