On April 15th, Microsoft will pull the plug on the Xbox Live service for all original Xbox games—which affects both Xbox 1 games played on both the original Xbox and the Xbox 360.
So say you downloaded an Xbox Originals game onto your Xbox 360. That game will still play just fine, but don't expect to go online with it. The same general rule applies to every vintage Xbox title no matter the grander Xbox platform.
But by cutting off these older titles (aka pissing off a bunch of Halo 2 fanatics), Microsoft will gain much more flexibility with the Live platform through announcements coming "in the next few weeks." [Gamerscoreblog via Kotaku]