One of the Exchange Server 2007 SP1 improvements is that we will be able to create mailboxes for more than one existent user at the same time. We can be following these steps to see the new feature:
Open Exchange Management Console
Expand Recipient Configuration
Click on Mailbox item
In the Toolbox Actions, click on New Mailbox
Select User Mailbox, click on Next
Now we have a new option on Existing users option where we can select more than one user to be created at the same time.
We can see a list in the same screen a list of all selected users.
In the completion screen, we will see that all users have been created.