Look, Sam Fisher is a master of espionage, so it's entirely believable the man could sneak his way into the packaging of Assassin's Creed II this week. Heck, now that his daughter is gone, he's thrown the rulebook out the window and part of his new anything-goes persona could very well be to synergize like he's never done before. Thus, this rumor that all copies of Assassin's Creed II on Xbox 360 will come with a download voucher for a Splinter Cell: Conviction demo.
One user over at NeoGAF claims a copy of Assassin's Creed II on Xbox 360 came with said insert, while further evidence from HipHopGamer shows what appears to be the alleged insert (it's since been removed from the original video; but Kouppa-Network Live still has it up). Logic points to the demo being the same portion of gameplay we checked out back at E3, though there's no evidence to verify that.
We've contacted Ubisoft for comment and will update this post when we hear back.
Source - NeoGAF post
Source - HHG video with insert reveal removed
Source - Pulled video showing insert