This day was inevitable, even if unfortunate. Microsoft is finally pulling the plug on XBOX Live support for original XBOX games. Starting April 15th, you'll no longer be able to hop onto XBOX Live for some nostalgic bouts of Halo 2 multiplayer.
I won't drone on here why I believe Microsoft shouldn't cut game support for a paid service (particularly for games they were selling days ago in digital form), but there are two sides to that argument. According to most stats, few gamers out there are playing XBOX original titles on XBOX Live. And, according to Microsoft, continuing legacy support for original XBOX titles is getting in the way of improving the Live service overall.