I was born and raised there so this touches me a little more than most of you, it is a sad day for the country of Poland and all Polish people around.
The Polish and Russian relations were always pretty intense in major part due to this: http://en.wikipedia.org/wiki/Katyn_massacre
This is why the whole delegations were headed to Russia in a first place, for a memorial of what happened in 1940, now there is another massacre that will be remembered on this day in the same place.
In Poland there is no vicepresident or so to speak, as soon as something happens to President the speaker of the house is in charge of the country and he appoints new election (the current speaker of the house was a favorite to win the election anyway).
It is beyond me however how they put some of the elite top leaders on the same plane, President his Wife and top 5 generals who were in charge of army, navy, air force and special forces were all on that plane among many other who lost their lives.