John, let me take a stab at these.
First, the 10-year window means that WebDAV will be supported in Exchange 12 from day 1 of its release to day 1+10 years. MS' standard lifecycle support policy is to support products for a minimum of 10 years after release. When Exchange 13 ships, if it doesn't include DAV suport, that won't make E12's DAV stop working.
Second, I read "work unchanged" to mean that DAV applications will be able to access the same set of properties that they now get, with no new ones added. However, since I think you can get pretty much any DAV property if you know the MAPI proptag, this shouldn't be an issue for Entourage.
Third, Entourage doesn't use any of the APIs that are being cut. (AAMOF, most of those APIs are pretty rarely used, which I suspect is one of the reasons they're being cut.)
Fourth, DAV is already a main protocol; you have to do something to turn it *off*. In Exchange 2003, IMAP and POP are disabled by default, but WebDAV is on by default because OWA depends on it. I don't expect that to change in E12 (but I don't know for sure).