The early plot ideas have him only revealing himself as a Sith just prior to his fight with Anakin and Obi Wan, but he was known to have left the Jedi many years before. Hope he doesn't wear that blue costume his bust is seen to be wearing whilst he fights. Not exactly Sith, was it? He'll look dead silly with that on and a flaming light scimiter.
Why not? I heard the shape referred to the handle only.
Anyway, it seems Dooku being a Sith will be revealed at the end of EpII and every unknowing viewer will be sooo surprised that he is evil. I guess he will be a very charming separatist with valid arguments against the Jedi Council. But when at the end he turns out to be a Sith called Tyranus, this is a big shock to everyone... I read something to this extent in the Insider interview with either McCallum or Lucas.
I like the idea of him being a charismatic kind of leader, and I had heard that he has a meeting with a few senators on Geonasis, in an attempt to get them to join his cause. The select pic of that senator at the round table suggests this too. From the rumours gathered already, it seems like there will be a lot of 'masked' evil in this film, which, with the exception of Palpatine, was missing from Ep1. It was like "Oh my God! There's a guy in BLACK with a red face and horns! He must be evil!". The idea of a fully human Sith with only a slightly spooky feel to him would be excellent.
Also, Obi Wan and Jango aren't at eachother's throats from the start, according to recent rumours. They are introduced by the prime minister of Kamino at Jango's appartment, but they would probably already recognise eachother from the (rumoured) Zam Wessel incident. Polite of them to wait until they were outside to kick off, though.