I feel like laughing when US-ers talk about Eastern Europe and try to impose their own pre-conceived notions of nation-states. The reality of more-or less fixed nation states dates back only to 1945, courtesy of FDR and Stalin, but before that talking about nation states in the same sense as Germany is a nation of the Germans, France is the nation of the French is nonsense.
Eastern Europe is essentially a multitude of ethnic groups cohabitating the same territory, sometimes coexisting sometimes fighting each other. These groups could be ruled by different state governments, Russian and Austro-Hungarian until 1918, and superceded by "national" government by name only between 1918 and 1939. A good model of Eastern Europe is Yugoslavia rather than France, Germany or even Belgium.
So one outcome of this ethnic mélange is plenty of ethnic prejudice across the board but also considerable inter-ethnic interaction, interrupted by occasional inter-group at one time or another. However, any attempt of linking these groups to nation-states nationalities simply distorts the reality that this was more of a highly localized, tribal-like violence.
As far as Poland is concerned, there was a lot of anti-semitic sentiment linked to the nationalist movement between 1918 and 1939, but I do not know of any organized collaboration with the Nazis to run the death camps. As far as I can tell, there only openly anti-semitic "resistance" organization during the Nazi occupation was the so-called National Armed Forces (NSZ), a rather small right-wing group often accused of collaborating with the Nazis, denouncing Jews, and attacking "communist" resistance groups. The extortion or denunciation of hiding Jews by private citizens (the so-called "greasers") for bounty money was also wide spread - but certainly not prevalent. I heard a lot of anecdotes about the "greasers" and other Nazi informants being executed by the resistance, but I do not know how widely spread that was.
Another factor is renegade groups of Russians, Ukrainians, Lithuanians, Latvians, Croats etc. who "switched sides" and collaborated with the Nazis and being used to do the "dirty work" (mass executions) which even the Nazis found repulsive. Of course, they were butchers, but they hardly represent nation states, as the US-esers and Westerners try to portray them.
So to summarize, I do not think that there is any justification in the claim that "Eastern Europeans" helped Nazis in the holocaust any more than pointing to the Nazi-organized Jewish police and saying that "Jews helped the Nazis to exterminate themselves." I understand, however, that history is often re-written for the sake of political expediency (as Finkelstein aptly noted of the "holocaust industry" in the US) - so I am pretty sure there is plenty of written accounts to the contrary.
Wojtek