It looks like Microsoft has discontinued its SideWinder Gaming mouse X8. It's quite a pity. This mouse was just introduced in 2009, with several advanced technologies. • Charger with magnetic connector. • Wireless. • Blue laser tracking. • Tilt scroll wheel. • 4000 DPI. For review, see: Microsoft SideWinder Gaming Mouses Review.
According to online rumors, Microsoft discontinued it not because of poor sale, but rather, too much competition from many companies that are dedicated to gaming devices. It appears that Microsoft has discontinued the entire SideWinder brand. (i wasn't able to confirm this 100%) Some of the models such as SideWinder X8 mouse are no longer listed on Microsoft's site, but they still sell some other models of gaming keyboard and mouse.