Microsoft are currently planning to change the casing of the Xbox 360 Slim 250GB from a glossy black finish to a matte finish, similar to the Xbox 360 Slim 4GB version.
Microsoft's Larry Hryb (aka Major Nelson) addressed fans about the change as certain news outlets mentioned that there will still be a choice between the matte and glossy cases come this holiday season.
Major Nelson stated, "We are actively transitioning all our Xbox 360 S consoles to a matte finish. The consoles will start to arrive in stores over the coming weeks, so if you really love the idea of a glossy console don't wait much longer."
Microsoft may be providing glossy cases in the form of their Limited Edition consoles, such as the Gears of War 3 Console and the Star Wars Console, but the glossy black console will no longer be available after a few weeks.
Source: Major Nelson