I was trying to help a customer open another person's mailbox after upgrading to Exchange 2007. The old way was to give administrator rights in System Manager in exchange 2003, but that no longer works with this new version. I went in and gave full permission rights in 2007 after hunting it down, but even that didn't work.
If you are an administrator trying to get access to everyone's email there is a command in Exchange Power Shell to do it.
get-mailbox | Add-mailboxpermission -User yourdomain\yourusername -AccessRights FullAccess
You still can't open it in Outlook using file- open- other user's folder, but you can add it as a separate profile. From there you can share the inbox or any other folder to open it in your profile later.