I found this article. Find solutions for crashes, conflicts and general problems with Entourage Number 15, midway down the page, has a paragraph entitled 'Identity disappeared from hard drive'. It suggests a problem with AV software I wouldn't have thought of.

Still thinking about the OFFICE 2008 IDENTITIES & OFFICE 2008 IDENTITIES BACKED UP folders... No idea what they are. I also just updated to Entourage 12.2.5 and I don't have them in my Identity folder. It's odd that your would have a folder called Office 2008 Identities inside a folder called Office 2008 Identities.