Dienekes posted some abstracts of the ASHG 2009 meeting. This one is in the category of facts we assumed but weren't totally sure of:
Abraham's children in the genome era: Major Jewish Diaspora populations comprise distinct genetic clusters with shared Middle Eastern ancestry Here, we present population structure results from compiled datasets after merging with the Human Genome Diversity Project and the Population Reference Sample studies, which consisted of 146 non-Jewish Middle Easterners (Druze, Bedouin and Palestinian), 30 northern Africans (Mozabite from Algeria), 1547 Europeans, and 653 individuals from other African, Asian, Latin American, and Oceanian populations. Both principal component analyses and multi-dimensional scaling analysis of pairwise Fst distance show that Jewish populations form a cluster clearly distinct from all major continental populations. The results also reveal a finer population substructure in which each of 7 Jewish populations studied here form distinctive clusters - in each instance within group Fst was smaller than between group, although some groups (Iranian, Iraqi) demonstrated greater within group diversity and even sub-clusters, based on village of origin. By pairwise Fst analysis, the Jewish groups are closest to Southern Europeans (i.e. Tuscan Italians) and to Druze, Bedouins, Palestinians. Interestingly, the distance to the closest Southern European population follows the order from proximal to distal: Ashkenazi, Sephardic, Syrian, Iraqi, and Iranian, which reflects historical admixture with local communities. STRUCTURE results show that the Jewish Diaspora groups all demonstrated Middle Eastern ancestry, but varied significantly in the extent of European admixture. There is almost no European ancestry in Iranian and Iraqi Jews, whereas Syrian, Sephardic, and Ashkenazi Jews have European admixture ranging from 30%~60%. Analysis of identity-by-descent provides further insight on recent and distinct history of such populations. These results demonstrate the shared and distinctive genetic heritage of Jewish Diaspora groups.
Remember that Fst is measuring variation, so more between group variation naturally indicates population substructure. The distinctiveness of Ashkenazi Jews is probably what I might term the "Iceland Effect", no matter your original genetic profile shutting off gene flow for centuries will naturally result in a random walk into unique territory which can't be explained simply as a combination of the founding population (in the case of Ashkenazi Jews I assume it was Middle Easterners and Europeans). Related:How Ashkenazi Jewish Are You?