UPDATE (20 Feb 2011): EURO-DNA-CALC has been superseded by the Dodecad Ancestry Project.
The current version is 1.1.1 Download it if you obtained previous version (1.1) which had a bug for deCODEme data.
A new version of the EURO-DNA-CALC has been released. The previous version (1.0) was described here.
Version 1.1 also uses deCODEme/23andMe data for an individual but outputs a maximum likelihood estimate of his admixture proportions from the three groups NW European, SE European, and Ashkenazi Jewish.
More comments on this dna-forums thread (registration required).
Below are the admixture estimates for Greg and Lily Mendel of 23andMe. An examination of the confidence intervals output by the program suggests that non-NW European admixture in Greg is a strong possibility, while Lily could conceivably have no non-NW European admixture. On the other hand, Lily's 10% Ashkenazi and only 1% SE European admixture imbalance is interesting. As with all genetic tools, the results should be interpreted in the light of other information about a person's ethnic origin and genealogy.