When I first installed Snow Leopard, over an installation of Leopard, it appeared to me that what happened was an Archive and Install (preserving users and settings) followed by migration of the old user accounts to matching new user accounts. All of this is done without user invervention and without using the term “Archive and Install" or “Migration Assistant."
Some of the problems people have had with losing user data when installing Snow Leopard were caused by errors that arose when the migration tool was used during this process. Those errors are recorded in the Install Log, but without particularly useful details.
