What is a good auto-generated key for a federated database?
One alternative is to perform a hash on some existing value or combination of values, for instance the user's login name and autogenerated identity (int):

SELECT 
    CONVERT(NVARCHAR(40), 
        HashBytes('SHA1', UserName + CAST(UserId AS VARCHAR)) ,2)
    AS FederatedIdentity
FROM Users


This way the value would be consistent across databases. You could persist the hash in a table; I usually create a view to work with generated columns like this.