NVIDIA 9500 GT + ATI 5850 seems to work on my work PC with 7x64 and using poclbm with --platform=xxx
One of the poclbm instances seems to hang randomly (it just stops with 0 KHash) but that might have something to do with the network here or some other shennanigan.
Can you detail how you did it and what the driver installation was like? Did you just insert all cards, install drivers, boot up with AMD card active, start mining with flag --platform=xxx, then switch to the nvidia card for display?