I note that Family Tree DNA uses the DNA tests/kits from the National Geographic Genographic project, and for an (apparently) additional fee will check the Nat Geo DNA result against their own database of other peoples' DNA. Since participants in the Genographic project would have to authorize release of any personal information, the identities in the FT database would have to be independently collected to provide any "personal matches."

The DNA Ancestry Project is less "up front" about what data they have to make useful conclusions from your personal results, but (in the manner of many geneo services) implies they can link you (but of course only you) to a number of famous ancesters.

Since the size of the database used for comparison is of "competitive ($$$$) value" quite likely you'd be required to let them put your personal info into their "commercial databases" so that when your long lost cousin Chongo looks for his relatives they'll be able to collect his 9 bananas before sending him to look you up.

There have been a few "popular press" articles published from people who've had various degrees of success with trying to trace/find individual ancestors this way; but for the most part the failures are in the majority, and the successes are marginally better than random chance associations. My own impression from the articles I've seen is "not quite ready for prime time."

John