The servers compromised were more like content servers though, not user data servers? Yahoo says no user data was accessed. But could the affected servers be used to more easily get at user databases from 'inside'?
Yahoo! keeps their user information mainly in a DB called 'UDB'... User Database. It is a key, value store and clients are only allowed to access permitted keys. So the encrypted user password, plain-text answers to 'secret questions' (for password rests), etc. were not accessed unless a login server was compromised.
For the user's data for each property (games, mail, etc) they have their own data stores, and those would have been compromised for sure.
