Apparently, RBR and Renault has officially filed a protest today for the double diffusers on the Toyota, Williams, and Brawn GP car. Both teams claim that they submitted a similar design for review to the FIA in early 2008 and was told that it's definately illegal. This is why they never pursued this path, even though most teams were sort of using the loophole in 2008. It was understood that the loophole was to be closed for 2009, and thus they were denied early on.
Renault has stated that the illegal diffusers give an extra 10% downforce (as mentioned in various articles over the last month), good for 0.5s/lap. A huge advantage.