This occurred in August 2007. The flag planter was Artur Chilingarov, who was deputy chair of the Russian State Duma at the time. They were having an election—to the degree that they have elections in Russia—and he actually descended 4,000 metres below the surface in a submersible and planted the flag himself, as a publicity stunt. As Alan Kessel told you, it had no more legal significance than the Americans planting a flag on the moon.
The good news here, from a Canadian perspective, is that in 1968 a team of Canadian scientists visited the geographic north pole on the sea ice. They actually dropped a canister to the bottom of the ocean that had a Canadian flag on it, so if we're talking about symbolism—and it's only symbolism—chronologically, Canada got there first.