Last Friday, Microsoft announced online support for original Xbox titles would be discontinued on April 15, but made no mention of disconnecting DLC servers earlier than that. Some forum users claim that the DLC servers for Xbox titles were taken offline as soon as the announcement was made last week. So, if you've continued to put off your infiltration of the Kola Cell in the original Splinter Cell, you'd better have already downloaded the mission.
[Thanks, Bren]