On this NAB video - http://www.youtube.com/watch?v=gg0VZWRy1NI, the Blackmagic rep mentioned Class 10 cards but he didn't mention the speed, so maybe 30MB/s cards might work for ProRes?
Just looking on the BlackMagic site and they picture 45MB/s card so I guess you were right about them being too slow. It might be more worth while getting an external recorder? But then you'd be losing the RAW option right? Unless there's a RAW external recorder?