Looks like this knowledgebase article may be helpful in your situation – good luck!
This does not apply for exchange 2007… the tabs in AD are no longer available with exchange 2007. This is a product that allows using the tabs again.
Are you running AD Users and Computers from your desktop? From what I have found with this same issue on ours we had to install the Exchange System Manager on our desktops to get the tabs to show up. Maybe a MS patch corrupted it and it needs reinstalled on your desktop?