For the past several weeks, I've been using Google's OpenSocial platform to develop a gadget that will work in the Orkut "container". I have developed several applications for Facebook in the past, so I can say from experience that compared to Facebook, this new platform has some very cool features. For example, developers have access to store shared data right on Google's servers -- that means you can build scalable applications that require no infrastructure of your own. To me, this feature makes developing on Google a little more attractive.
Google's planned launch date was, and still is November 5th -- though the date may slip back a day or two. Currently, only Orkut is supported as a "container", but that is just the beginning. You will soon start seeing these gadgets appear on other properties such as iGoogle. The API and Orkut sandbox I've been using to date is still not public, but you can expect them to be available on or around the launch date. Tonight, the New York Times posted an article about Open Social that you may also find interesting.