Exchange looks up data on the global catalog server(s) only. To see the changes immediately in AD Users and Computers, choose the Global Catalog server that your Exchange server is using to make changes or manually replicate to the other Domain Controllers in AD Sites and Services.
Exchange 2007 management moved from Active Directory directly to the Exchange Server. This is also why you cannot successfully create a fully functioning mailbox for Exchange 2007 in Active Directory either. Create mailboxes and distribution groups using the Exchange Server Management console only. Do not use Active Directory for these functions any more.