I'm assuming you're talking about this <
F-Secure> blurb last week? As it points out, the purpose is to hide the true extension so they will open something that will then install some malware. Apple's Gatekeeper could even let this problem file through, without warning, if it had an Apple approved, developer signature. I think this is another reason to keep the entire file name/extension visible in Finder (Prefs->Advanced->Show all filename extensions). That way, you won't even have to depend on Gatekeeper to warn you, you can actually see the double extension text, even if it's not backwards!
"What you
don't see
can hurt you!"