Juza from juzaphoto did a side-by-side comparison a couple of years ago. I can't find the link but IIRC he concluded that the IQ was identical when shot at the same aperture.
I don't doubt their results but I have shot these side by side for over a year, and maybe it is a difference in the copies I have but in my experience they have been different. (Same different results on both my 7D and 5DII.) Of course my tests have not been scientific at all...I take indoor sporting pictures and depending on where I am and what the conditions are I bring them both. It has been a lot of fun to experiment. I enjoy the pictures from the 85L more...of course that could be a placebo effect knowing I spent $2k on it. I would also question how much better given the large cost difference...the 85 1.8 is a great lens too and I was always happy with it before buying the 85L. I just could not resist the the examples of the 85L I saw for before deciding to purchase it.