If you are in a country which hasn't heard of Orkut, let us inform you that this Google-owned website is the most used social network in Brazil - with a significant number of users in India too. While it may be small compared to Facebook (which boasts of half a billion users), 52 million active Orkut users is, by no means, a small number.
Anyway, as for the Bom Sabado worm, it is a JavaScript based worm that spreads itself through Orkut scraps (the Orkut equivalent of a Facebook wall scribble). The worm is an auto generated message which might look like just another scrap from your buddy. The scrap actually embeds a malicious JavaScript code which infects the profile of the person as soon as he opens the scrap page. Soon after this, the code is executed and it will make the user join bot communities automatically. The worm then starts posting similar scraps to the user's friends using his name, thereby spreading itself. The worm also steals browser cookies.
Orkut has now posted an entry