Matthew Byrd has explained an important issue in the Exchange SErver 2007: the Certificates for the web services. In the Exchange Server 2007 we have a bunch of services OAB, autodiscover, rpc over httpd (formely oUtlook Anywhere), pop and imap that can use SSL.
We have to concern about the SSL for internal and external users. At the end of the MsExchangeteam.com post we can see a matrix explaining which method and with their respective pros and cons.
With Exchange 2007 we introduce the idea of the Autodiscover service. This service allows your Outlook 2007 clients to retrieve the URLs that it needs to gain access to the new web services offered by Exchange 2007. These web services (OAB, UM, OOF, and Availability) provide a good portion of the new functionality available to Outlook 2007. Please see this blog post for more details on Outlook 2007 feature matrix based on the Exchange server version.
Read more at: http://msexchangeteam.com/archive/2007/04/30/438249.aspx