Its a common failing due to poor splash gaurd design on the front brakes. IIRC there is a larger sheild that you can use thay encases more of the rear of the disc to preven them getting too much spray on them.
I used to have this withe the 280s, but wehn i went 288, i also changed the shields and that cured it,it also stayed away when i went 312, as i kept the same shields.
ThanksI have no gaurds front or back, never had a problem with wet braking because of it
that said I have some RS4 brake guards if you want them, yours for postage!
I experienced this last night on the way home from London in hard rain/spray on the M25/A1 - It can be a little disconcerting at times!
I've a theory on this
As we've just come out of a particularly bad winter we've been gentle on the brake pedal trying to keep the wheels from locking. Now the weather has picked up we still are being quite tentative with the pedal, slowing the rate at which the film of water is cleared from the surface of the disc.
I could be wrong of course but the next time it rains I'm going to stamp on the brake pedal and see what happens