Exchange 2007 does not support ExMerge like Exchange 2003 and earlier versions did.  To centrally import mailboxes, do the following:

1.  Download and install Windows PowerShell onto a 32-bit workstation.
(http://www.microsoft.com/windowsserver2003/technologies/management/powershell/download.mspx)

2.  Download the 32-bit version of Exchange Server and install the management tools.
(http://www.microsoft.com/downloads/details.aspx?familyid=6BE38633-7248-4532-929B-76E9C677E802&displaylang=en)

3.  Install and configure Outlook 2003 SP-2 or later.

4.  If the PST files are on a network share, then map a drive letter to that share, such as P:\PSTFiles

6.  Grant Send-As and Receive-As rights to the user who is doing the importing.
    Run the following command from the Exchange Management Shell, replacing the "My" variables with the correct values:
      Get-MailboxDatabase -identity "MyServerName\First Storage Group\Mailbox Database" | Add-ADPermission -user "MyDomain\MyUserAccount" -ExtendedRights Receive-As, Send-As

7.  To import a single mailbox, replace the value of UserAccountName as needed and run the following command:
      Import-Mailbox -Identity UserAccountName -PSTFolderPath P:\PSTFiles

8.  To import ALL the mailboxes in a folder:
       Get-Mailbox -Database 'MyServerName\First Storage Group\Mailbox Database' | Import-Mailbox -PSTFolderPath P:\PSTFiles


Credit goes to the Microsoft Exchange Team blog for pieces of syntax used in this tip.
http://msexchangeteam.com/archive/2007/04/13/437745.aspx