Yes, Operation Vistula was polish action, but remember, we was then under communist rule, and many of polish families suffered after resettlement their relatives and friends from childhood to another country.
Operation Vistula and the expulsions of Germans, along with similar actions, basically defined the current nation state in Eastern Europe. Ethically they were wrong, forced resettlement and all, but I can't say that they didn't lend to stability. There was so much nationalism during and after the war that it probably worked out that most ended up amongst their ethnic kin.
Also, back to Davis's point, the "unnatural" Poland comment has me kind of irked now. Post 1795, practically every Slavic nation, aside from Russia was not sovereign until the USSR's fall. And Russia was only "sovereign" since all the shots were being called from Moscow. I think Slavs have yet to find their place and "natural" state of being. 20 years of freedom (more or less) is not really that long.
I also must confess to a West Slav centered bias in that last statement, I do not know that much about the history of Southern Slavs, but they were under Austria-Hungary for a long time.
Lastly, could it not be said that with the fall of the USSR, a new chapter in history arose? One where Slavs really have the chance to now forge ahead with truly their own will?