Obviously, the Russians can't bomb the US or even Canada with only two unescorted bombers, so the point of these little adventures is symbolic. Russian had suspended these fly-bys after the end of the Cold War but began them again in August of 2007. Most likely Russia is feeling its power weakening and is trying to reassert itself as a major international actor, but to me at least these gestures seem like a symbol of growing insecurity... if Russia was really that powerful, it probably wouldn't feel the need to constantly remind people that it was powerful.
The Canadian Defense Minister stopped short of accusing the Russian's of plotting any kind of symbolism by the event, and a Russian air force spokesman went on record saying that Canada's statements are a "farce". But considering that Mr. Medvedev timed his announcement of Russia's deployment of surface-to-surface missiles to the border of Poland and Lithuania on the same night that the US presidential elections were announced, I doubt that this latest round of saber rattling is a coincidence.
The missile defense shield in Poland is still a point of contention and quite possibly the impetus for this event. Russia perceives the potential missile installations as interference in its traditional sphere of influence. Maybe its trying to remind the US what that feels like.