One thing that I found is a bit frustruating the lack of documentation regarding Entourage connectivity to Exchange mailboxes in co-existence scenario.
The first challenge actually happened a few weeks when our team has deployed Exchange 2007 CAS farm to replace our existing Exchange 2003 FE. We found that by default when deploying CAS role, it will enable legacy virtual directories to support OWA for exchange 2003 mailboxes but not WebDAV to support Entourage. Thus, our team had to set it manually via GUI or ADSI IIS provider in powershell. Big thanks to Nathan Winters and his article to elaborate on this.
The second challenge came when a mailbox was migrated to Exchange 2007 CMS. Entourage 2004/2008 started to display "Unexpected Error (170)". My initial research kept pointing me to KB947802, however that was not the problem as I verified that HTTP protocol settings exist in AD. The actual solution to the error message ended up KB931350. It appears Entourage users must now enter a fully qualified URL to their mailbox https://firstname.lastname@example.org. This does not look too apealing at all. It's time to change the user docs!