The New Scientist is running an interesting story of a boy who managed to identify his biological father, an anonymous sperm donor, by getting his own Y-chromosome sequenced and then hitting the internet.
He started by paying a service, FamilyTreeDNA.com, to sequence bits of his own Y-chromosome DNA. Since the Y-chromosome is handed down father-to-son, this DNA could have come only from the sperm donor. The service got him in touch with two other men with very similar Y-chromosome content to his. Those two men had very similar last names with a minor spelling difference (it took me a minute to remember that last names are also frequently patrilineal, thus serving as a real-world tracer of the Y-chromosome).
He used the sperm donor's birthdate-- which his mom knew-- to query a different database, Omnitrace.com, for every male born in a certain place on that date. Only one of these guys had the last name he was looking for.
As New Scientist remarks, "The news will be especially unsettling for men who donated anonymously before the power of genetics was fully appreciated. Donors were often college students who traded their sperm for beer money. Many have not told their wives or children and have never considered the implications of having a dozen offspring suddenly wanting to meet them."
I'm not at which step in this search the anonymity of the sperm donor could have been defended. Are you still cool with filling out information at the supermarket?