[Additional update: further commentary by Ryan Naraine at http://www.zdnet.com/blog/security/koobface-for-mac-os-x-squirming-on-facebook/7579, by Dan Goodin for the Register at http://www.theregister.co.uk/2010/10/27/koobface_for_mac/, and by Casey Johnston at http://arstechnica.com/apple/news/2010/10/new-java-trojan-attacks-mac-os-x-via-social-networking-sites.ars.]
[Update: @stevejoblard also tells me that there's been discussion in Apple support forums on OSX/Koobface since early October. I'm afraid it's a while since I spent much time in such corners of the Internet: too much Windows action... Jerome Segura of ParetoLogic has also blogged on the topic and indicates that the attack works quite happily on Linux, though I'm still unsure as to some of the details.]
Tip of the hat to @stevejoblard for bringing my attention a post on the Intego blog about what they call OSX/Koobface.A. There’s a cryptic reference to “Reports [that] have circulated discussing a Trojan horse, but without understanding either the scope or the functioning of this malware” which I suspect refers to the SecureMac post referenced here.
Don’t drag me into this, boys: I haven’t seen a sample yet.
Intego’s report is at http://blog.intego.com/2010/10/27/intego-security-memo-trojan-horse-osxkoobface-a-affects-mac-os-x-mac-koobface-variant-spreads-via-facebook-twitter-and-more/, and is characteristically comprehensive (and, no doubt, to Intego’s usual high standard of accuracy).