Hmm, on one hand, not much to gain from killing Pres of Poland (no ongoing disputes and NATO missile shield isn't going to disappear if you kill the one guy – in fact, you might guarantee his brother's election next time around).
On the other hand – heavy fog, no instrument guiding system at a military airport, numerous attempts to land, plane full of senior people used to getting their way and "don't you know who I am-itis", and two pilots being prodded to land so they don't miss the big ceremony the next day.
I'm not saying Putin wouldn't do it cos he's a good guy, just that there's no real incentive. Besides, there's a putsch to be managing in Kyrgyzstan right now.
But having said that, I'm surprised they had the plane serviced in Russia. How would you know it wasn't filled with listening devices?