I'm sure Rossi will not want to end his career on an bike in which he is not 100% sure he will be competitive. He is very smart in his moves.
I believe this is more like an ultimatum to Yamaha "Drop Lorenzo or else". He wants to pressure Yamaha to not renew Lorenzo. He doesn't have the intention of moving to Ducati. He just wants Lorenzo out.
Rossi left the best bike in the paddock (Honda) to go to the very uncompetitive Yamaha. He looked at Ducati first but didn't get on well with Suppo and how he ran the garage. Burgess didn't agree with how Suppo developed the bikes either. Burgess is an integral part of Rossi's success and Rossi will not likely move without him.
Ducati knows that Rossi won't work with Suppo and as such Suppo is now gone, off to Honda. (It's likely that the separation was mutual)
The Rossi/Ducati deal has not only been all over the european media, it was being discussed by the MotoGp anouncers during the Sepang live feed.
When Rossi was asked about a move to Ducati, instead of answering with the usual remarks, he said "I'm contracted to ride for yamaha through the 2010 season", sounds a bit diversionary to me.
One thing that has been floated around is the concept of a one off factory team similar to the Nastro Azzuro team that was assembled for Rossi when he first came to the premier class. Granted I think that Rossi will stay with Yamaha if Lorenzo goes, I won't put it past him to make the switch to Ducati.
Of course Lorenzo seems to have lost the heart of his crew as his crew chief has fled to tech3, and his electronics engineers are moving to Honda. could this be an omen? Let's face it, anyone who writes an autobiography at 20 must have a huge ego, it's possible that it may be a factor there.
If Ben shows that he has what it takes to run with the aliens (todays performance is a strong affirmative) then that may cut into Lorenzos' negotiating ability some too. Either way you slice it, Rossi is playing with fire, but at least he has an escape route planned.