(One thing to point out is that, in the past, these types of sites—very common vectors of Windows malware—only delivered Windows .exe applications. The fact that such a site is providing a Mac rogue antivirus is new, and extremely rare. While the site itself still shows a fake Windows screen, the rogue antivirus itself is a well-designed Mac application.)This application is very well designed, and looks professional. There are a number of different screens, and the grammar and spelling are correct, the buttons are attractive, and the overall look and feel of the program give it a professional look.
A more detailed write-up than the one offered by TUAW is availaible from Intego's "The Mac Security Blog" here.