Microsoft Knowledge Base Article 250455 contains the following summary:
When a new user is created in Active Directory, the
field is always generated in FirstName LastName format. In turn,
this field sets the
field on creation,
you end up with a FirstName
LastName formatted global address
You can make this change by using the Adsiedit utility. Adsiedit not only changes the default way the Display Name field is built, but also the Full Name (that is, the "cn") field, therefore, users appear in the chosen format when you look in the Users and Computers snap-in.