There's a lot of fuzz in the genetics blogosphere about a new paper by J. Novembre and M. Stephens, titled Interpreting principal component analyses of spatial population genetic variation (Nature, 2008). I have only access to the abstract (sorry: too poor to subscribe to anything) but p-ter at GNXP and G at Popgen ramblings, for instance, have been discussing it. The latter says:
These results do not to say that human populations did not expand out of particular regions, just that PCA maps are not the best tool to judge this.Often we find the opposite interpretation, beginning by the classical works of Cavalli-Sforza himself. So, please, be very cautious about PCA interpretation: they may not show what it seems on first sight - after all they aren't but complex mathematical abstractions, not the real thing.