Since I’m running my WordPress blog with strict XHTML served as application/xhtml+xml, and since the injected code isn’t valid XHTML, I wouldn’t’ve missed the breakage when upgrading WordPress if it was there, and I only have one configured WordPress instance on my server. I keep my WordPress install up-to-date with stable releases via svn.
Unfortunately I don’t really have enough other than this to create a new ticket, but I’m fairly sure that this is the only WordPress install I have, and that the post was modified after upgrading to 2.3.1.
