Street Fighter X Tekken could have had Xbox 360 exclusive characters. The PS3 version of SFxT will have Cole McGrath from InFamous and Sony cat mascots, Toro and Kuro. According to the game's producer Yoshinori Ono, Capcom was in discussions with Microsoft but they ultimately never came to an agreement.
"We do have the exclusive characters for the Sony platform. We have Cole, the two Sony cats, Kuro and Toro. But basically for the Xbox 360, we were in discussions with them [Microsoft] for which character to put in as an exclusive, but we weren't able to decide on a character because of differences in timing and things like that," Ono told Xbox360Achievments.com.
"So unfortunately for Street Fighter X Tekken, only the PlayStation family will be getting the exclusive characters, but it's something we wish could have happened. It's just too bad, basically."
"In terms of characters we wish we could have put in the game, Master Chief or Marcus Fenix from Gears of War would have been really cool, but it was just really difficult," Ono said. "We have Cole from inFamous on PS3, which is really cool, but that's just the way it turned out."
This reminds me of what happened with Mortal Kombat. The PS3 version had Kratos while the Xbox 360 version didn't have any exclusive characters. These seem like missed opportunities to promote a platform, sell a game, and to pay fan service to your playerbase. Hopefully Microsoft will see the error in their ways and finally just add Horstachio from Viva Pinata to fighting games.
Source: Xbox 360Achievments