James Nickel's essay on treads largely uncotroversial ground in its examination of the moral pitfalls of ethnic cleansing, but perhaps his somewhat guarded explanation of the grounds for permissible instances of ethnic cleansing offers a more interesting field for discussion. Nickel provides a series of criteria for evauluating such instances and suggests that certain forms might be "morally tolerable as a way of dealing with severely deteriorated situations".
In truth, Nickel (probably due to the senstive political context of the Bosnian War at the time of publication) probably gives short shrift to the effect of forcible population transfers in favor of political stability. The single largest such transfer occurred after the Second World War, as (among others) millions of Germans were expelled from Central and Eastern Europe and driven destitute into Germany proper. This was undoubtedly a brutal process, but it was essentially endorsed by Churchill and Roosevelt as an expedient to stable postwar political boundaries (Hitler, after all, had used the plight of the Sudeten Germans as a pretext for his annexation of Czechoslovakia). Indeed, the remarkably peacable state of modern Europe can be explained in large part by the relatively homogenous character of most of its nation states. By contrast, it's cliche to note the arbitrary, trans-ethnic borders of postcolonial Africa are responsible for much of the political instability there.
To highlight a more specific example, a Mideast peace settlement leading to the establishment of a Palestinian state (should that ever occur) would almost certainly require forcible population transfer from some of the Jewish settlements in the West Bank, as already occurred in the unilateral Israeli withdrawal from Gaza. Given that this policy is essentially what most of the international community is demanding, it calls into question the moral objection to population transfer per se, rather than merely its nastier manifestations.