It has been done, but as stoanee said it ain't easy.
Found this thread;
Folding Forum • View topic - ATI 4870 & Nvidia 280,
toTOW posted that another user reported success in a post from a French language site here;
FAH-Addict : Sujet - ATI Radeon + NVIDIA Geforce !!!.
My French isn't good enough to confirm this, YMMV...