These are the first reasonably precise measurements done of Canon CMOS Quantum Efficiency that I've seen:
http://astrosurf.com/buil/50d/test.htm
Whats it all mean? In a nutshell, the Canon CMOS sensors are 1/5 as sensitive as the best monochrome sensors. This difference is mostly due to the bayer colour filters which cover the pixels. I guess we all knew that but its interesting to see the hard data for once!
Interestingly the new 5D is quite a big improvement over the old 5D.
Terry