Microsoft have grown tired of the restrictions the "old version" of Xbox Live is putting on the current one, and they have now decided to shut down the servers for the people still playing on the old Xbox-machine (which includes emulated games on the Xbox 360). Major Nelson has stated on his blog that this part of Live will be shut down on April 15, and that it opens up for more development of new features for the Xbox 360 Live service.
So if you're still playing Halo 2 or Star Wars: Battlefront II, the two most played games on the original Xbox, online this won't come as very good news.
For me, it means about two months to play Steel Battalion II online! Or not.