If you are not familiar with what OPATH is, then you must read these two blogs when you are done reading this one!!! OPATH is basis for the filtering syntax now used by The Microsoft Management Shell (Powershell), and is therefore the filtering syntax used by Exchange 2007. It now replaces the complicated syntax of LDAP used in Exchange 2003, and will allow for filters which are easier to create and interpret.
Evan Dodds from the Exchange Product Group has written two hard core OPATH blogs which have been posted on the Microsoft Exchange Team Blog. I highly suggest that you ready both of these as they contain a wealth of information!!