By now, you may have read stories regarding one of Sega's spin-off companies called Overworks reportedly signing on with Microsoft as a third-party developer for the Xbox console. Overworks, formerly Sega's AM7 R&D division, is currently working on Dreamcast titles such as Skies of Arcadia and Sakura Wars 3.
The company recently responded to the reports, saying, "No, that is not our company. That [Overwork's] is another company from a different country. If you look closely at the name, there is an apostrophe." Despite the rampant speculation that the Sega spin-off companies will develop for multiple platforms, all of the firms have only announced Dreamcast projects to date. We'll have to wait and see if any of Sega's spin-off companies eventually sign up to develop for the Xbox.
On a side note, Artoon, a company consisting of former Sonic Team members, has signed with Microsoft as a third-party Xbox developer. The company is currently developing Pinobee for the Game Boy Advance.