As expected the FIA, probably most interested in not contradicting itself, declared the Brawn, Toyota and Williams diffusers to be legal.
What was not expected and a big disappointment was to hear Ferrari's lawyer refer to Ross Brawn as "a person of supreme arrogance" for essentially coming up with his own, clever interpretation of the rules as written. Compound that with the apparent argument made by Adrian Newey that these diffusers are unsafe because they make cars too fast and the whole thing just looks
bad plain stupid from the outside.
As a counterpoint of course, this ruling goes completely against the FIA stated intentions of cutting costs as many teams will be forced to spend huge amounts to rush new parts onto the cars. It was rumored Mclaren spent a few million on a front wing just for Interlagos last year, imagine what a complete re design of the rear aerodynamics of the car must cost.
Also the 2009 rules arise from recommendations made by the Overtaking Working Group which was made up by designers from Ferrari, Mclaren and Renault. It is understandable why those three teams made the argument that there was an infraction against the spirit of the regulations and it is understandable why their cars were not designed the same way. However proving infractions against spirits never worked very well and good luck getting a ruling that would have annulled what has been a dream media story again for Formula 1, right at a time when the sport so needs to be at the center of attention. Not to mention stifling that essential element for technical progress: creative disregard for "the way it's done".
For one interpretation of how the Brawn diffuser works look HERE