Several leaders will be in Warsaw to commemorate the start, most notably Angela Merkel and Vladimir Putin (who, I hear tell, rented out the entire Grand Hotel in Warsaw.)
The Poles are demanding an apology and acknowledgment from Putin (having gotten several from Germany over the years.) Putin seems to deflect these, giving vague statements about the Molotov-Ribbentrop Pact, mostly saying that it was 'unfortunate', but standing by the Russian view that it was just as valid as Chamberlin's "Peace in our time" Munich Agreement, which handed Czechoslovakia over to Germany, and delayed the war by about a year. There's one big, stark difference between the two: England and France didn't then conspire with Germany to carve up much of Europe and start attacking countries (the USSR did just that, annexing the Baltic States; fought Finland to more-or-less a draw; annexed a half-dozen other countries.)
Six years later, Poland was a smoldering pile of rubble with about a fifth of its population dead. For the next forty-five years, they would be lorded over by that wonderful Union of Soviet Socialist Republics.
Attitudes in Western Europe and America are a tad different than here. Most everyone there is eager to forgive and forget WWII and the Cold War, but in Eastern Europe, they're still at each other's throats (not that Russia has done much to satisfy.) Putin isn't much into acknowledging that Russia has faults, or has done any sort of wrong at any time. Drunk as he may have been, Yeltsin at least laid a wreath at Katyn.
Well, there's still some anger here. I had a particularly rousing discussion with one of my students about Polish history. She immediately dismissed that because I was an American, I knew nothing of history, nor could I even have the capacity to imagine what happened to Poland. I suggested she start liking the Germans, seeing as they are mostly decent people, and now are one of Poland's allies and benefactors. She cared not.
Also something further, I find this interview very interesting:
Richard Overy has written a most excellent book The Dictators: Hitler's Germany, Stalin's Russia. It's superbly written, very illuminating, and surprisingly funny. I recommend it to anyone who wants to learn about Hitler, Germany, the USSR, etc.