If you forgot to do this during setup, or you just plain didn't know what the external name was going to be and now addressbook lookups don't work from the outside, there is hope!

LCSCMD to the rescue. You may find this under

\Program Files\Common Files\Microsoft Office Communications Server 2007\

The command line for changing or updating the reverse proxy FQDN is:

Lcscmd /web /action:updatepoolurls /externalwebfqdn:externalreverseproxyurl /poolname:yourpoolname

where externalreverseproxyurl should be in the form of external.contoso.com and yourpoolname  is the 

name of the OCS standard server or internal poolname of your OCS enterprise deployment.