This move will extend to Xbox 360 owners, as well, meaning that original Xbox games that run via the 360's backwards compatibility feature as well as digitally downloaded Xbox Originals will also stop functioning online on 4/15. Offline play in games for the original Xbox should continue without issue, so feel free to break out that copy of Tao Feng: Fist of the Lotus that I know you're sitting on.
The reasoning behind this move hasn't been completely spelled out, but the gist of it's right here:
To reach our aspiration, we need to make changes to the service that are incompatible with our original Xbox v1 games. We will contact the Xbox LIVE members directly impacted by this change and if this includes you, I encourage you to check your LIVE messages and associated e-mail account over the coming weeks for more details and opportunities. We view you as a partner in this process.
Whitten goes on to say that more details will be available "soon." Last year, Patrick Klepek over at G4 reported that continuing support for Halo 2--by far the most popular online game for the original Xbox--was preventing Microsoft from raising the limit on Xbox Live friend lists. Microsoft hasn't confirmed whether or not this is, in fact, the case. Currently, the list is capped at 100, as it has been since the service's launch back in 2002.
I don't think I've touched a game for the original Xbox since 2006. How about you?