As discussed earlier I received a long list of warnings and errors when running the premigration check. The next entries in my list are a couple of warnings of this type:

All users listed in these events were accounts which are not available on my MOSS 2007 test server - either because they are in a different domain (my test server is not yet joined to a domain) or because they are local machine accounts. As I did not want to install MOSS 2007 beta 2 on my production MCMS server machine the local machine accounts do not exist on the MOSS 2007 box.

The event details explain what happens when the migration tool identifies a user in the repository which it cannot find:

These users will not be migrated.