The conclusion of the authors of this paper on Sorb genetics is that these last Slavic speakers of Germany are closer to Poles than to Germans in terms of overall genetic structure. This is a reasonable conclusion based on the data they analyzed, but the problem is that they didn't analyze any German samples sourced specifically from near the Sorb homeland (ie. eastern Germany). So this paper probably has things right, but no one can really say for sure. It's good to see that the authors are aware of this issue...
One caution regarding our results is that the geographical origins of our reference populations are crudely characterized only by country and thus may not be random samples. If many of the Germans in the POPRES data are western German samples, this may inflate the apparent differences we observe between Germans and Sorbs. The LPZ Germans contained two individuals from Eastern Germany who do appear closer to the Sorbs, suggesting that population structure within countries is a valid concern. Certainly, a tighter and denser sampling of German, Polish and Czech individuals from regions surrounding the Sorbian territories would be ideal for confirming or refuting the results found in this study.
Just in case there's a re-run being planned, please remember that the Poles living close to the Sorbs are actually of eastern Polish origin, from the former Polish territories in Belarus and Ukraine. It would be better to sample western Poles living in areas that belonged to Poland prior to the Second World War.
Krishna R Veeramah et al., Genetic variation in the Sorbs of eastern Germany in the context of broader European genetic diversity, European Journal of Human Genetics advance online publication 11 May 2011; doi: 10.1038/ejhg.2011.65