Interesting question and I'm glad you asked.
A man once told me you're only as good as your last win. This may be true in most sports, but in Formula1, you're only as good as the new parts due to be bolted on your car, here in lies the big question mark. As more teams add double diffusers, it will be interesting to see who has the pace in the sandy Bahrain desert. In Shanghai, Red Bull, Brawn and Renault dominated qualifying whilst it was clearly Red Bull's day in the very wet race.
Red Bull seems to have the most mechanical grip available on their car at present, which unlike aerodynamic grip, favours wet conditions. Think of mechanical grip as what the suspension and tires do to keep a car on the road, aero grip comes from air travelling over the car pushing it down to the ground.
Red Bull's high mechanical grip explains why they are the only non double diffuser team to be performing well, the RB5's mechanical grip is superior to the diffuser cars in turn balancing their respective performance advantages out.
Unfortunately for Red Bull, it doesn't rain often in Bahrain. Expect to see the two Brawn's near the top of the tree followed closely by Toyota, Glock set fastest lap in Shanghai.
Based on the respective success of updates, I have a strong feeling we'll see Hamilton and Alonso up near the top. Should Alonso decided to qualify light, as in Shanghai, there's no reason why he shouldn't perform well.
The McLaren, also relying on aero grip should do better with new double diffusers in Bahrain. Bahrain is a circuit with fast straights and slow corners, so you might find cars relying on aero grip and will have lower straight line speed.
Where Ferrari lies is anyone's guess. They have performed poorly to-date but with their huge development budget, they may have a double diffuser ready for Bahrain and start to be competitive again.
So who do I think will win?
I think it will be the very smooth Jenson, followed by Barichello, Alonso, Vettel, and Webber.