Specifying A Mailbox Database Is Optional In Exchange 2010...

A mailbox database has to be specified while creating a new mailbox user in Exchange 2007 Server whether you use the console or shell. If you don't select a mailbox database (even if you have only one), the wizard will prompt you to select a database.

In Exchange 2010, this behaviour has been changed. You can specify a mailbox database while creating a new mailbox user. If you don't choose a database, the system will select one for you. (depending on the load on each databases I would think!)

Have a look at the snapshot to make things clear...