(&(objectCategory=user)(objectClass=user)(!(MSExchHideFromAddressLists=TRUE)))
Odd you need to define this, by the sound of it, the string is to make a user not searchable. According to everything MS has to say on it, the attribute of True for this is to take an employee who is no longer in the system out, for instance, on Maternity leave. Then when they come back you set it to "False" and they are back in. Seems like it is not working server side. But I'm not an LDAP expert.
I made my suggestion using these sources, it is untested :
http://technet.microsoft.com/en-us/library/cc261947%28office.12%29.aspx
http://community.spiceworks.com/scripts/show/1802-hide-all-disabled-users-from-gal
http://community.office365.com/en-us/f/148/t/200682.aspx
Hope that helps