Russian authorities say early evidence indicates the explosion was, indeed, a terrorist act:
Russian state TV, citing Russian authorities, said the explosion was the act of a suicide bomber who stuffed a homemade bomb with small metal objects to make it more deadly, then activated it in a crowded area where many people were preparing for their flights. CNN could not independently verify those claims.
Chechen rebels, who many suspect carried out today's atrocity, have claimed responsibility for a series of terrorist attacks in recent years, including a Moscow subway bombing that killed dozens of civilians last March. According to a 2005 report from George Washington University, more than 93 percent of Chechnya's population is Muslim.
Responsibility for today's blast remains unclaimed for now.
UPDATE - How did the suicide bomber(s) breach airport security? It appears they didn't:
Russia’s Novosti news service cites eyewitnesses claiming “two terrorists blew themselves up as passengers emerged from the international arrivals zone.” If so, then they picked the softest target that airports have: a place where lots of people congregate, in anticipation of their loved ones’ arrival, without passing through machines to detect chemicals or metallic objects.