From XboxOZ360:
"First there was GTA:IV's Exclusive DLC, then we discovered Fallout 3 will have Exclusive DLC and now Tomb Raider: Underworld has joined the ranks of those games offering Xbox 360 owners exclusive content through XboxLIVE.
The two majors have both stated publicly that the era of "Exclusive titles" - 3rd and some 1st party titles, has run it course, due mainly to a change in the distribution and financial turn around on games in the current market. Not to mention major publisher/developers like Capcom dropping exclusive content."