|MadSci Network: Genetics|
The research you mention used mitochondrial DNA (mtDNA) to investigate the origins of domestic dogs (Savolainen et al. 2002). Since mtDNA is always inherited from the mother it can in principle be used to investigate whether a particular woman (e.g. Sally Hemmings) was a maternal ancestor of someone alive today. This would require DNA samples from the living person and a known maternal descendant (i.e. a daughter of a daughter of a daughter...etc.) of the possible ancestor. More details about how this can be done can be found in the book "The Seven Daughters of Eve" by Bryan Sykes.
In the Jefferson case the question is not whether the individuals are descendents of Sally Hemmings (this is already known) but are they also descendents of Thomas Jefferson? To answer this question we need a molecular marker that is always inherited from the father in order to trace paternal ancestry. The Y chromosome has been used for this purpose (Foster et al. 1998). This can be used in a similar way to mtDNA, although only in males since females don't have Y chromosomes. More details can be found in the book "Y: The Descent of Men" by Steve Jones (not yet published in the US).
To return to your original question, if you want to know whether a certain individual is a descendent of Sally Hemmings then you could indeed use mtDNA, but in order to do this you would also need to obtain DNA from an individual with a direct link back to Sally Hemmings through the female line.
There's a lot of information (and misinformation) and debate on the web about the Jefferson/Hemmings question so try searching google for more details. But beware, not all of this material is very scientific so be sure to check out the original sources for yourself.
Try the links in the MadSci Library for more information on Genetics.