See Bolnick et al's The Science and Business of Genetic Ancestry Testing for a bit on the science and its limitations generally.
See Royal et al's Inferring Genetic Ancestry: Opportunities, Challenges, and Implications for more background on genetic ancestry testing companies.
See my chapter, DNA and Native American Identity, published last year in a National Museum of the American Indian Anthology, indivisible: African-Native American Lives in the Americas, ed. Gabrielle Tayac. Washington, D.C.: Smithsonian, 2009.