The Exchange server Admin can view the Mailbox Resources under Information Store on the server - this will show number of messages and total size of mailbox (not, unfortunately, a count of read vs unread messages), as well as a last logon time and details of the particular client that last connected. It's not brilliant, but it at least can show you which mailboxes haven't been logged into in a long time.
(NB this last connection information is broken if you have any kind of mailbox maanger or antivirus software running against the server that does MAPI logons to the mailboxes, as this info will effectively mask the real user connects)