my personal experience causes me to question whether sensor size or tecnological advancement is at the heart of this IQ debate. i shot a great deal with 12mp full frame 5d coupled with first rate zeiss lenses. i loved the IQ and felt no small sensor cam could touch it, and that was backed up by the results i got (get) from my ep2.
then i got a 12mp aps-c x100. frankly it performed as well as my 5d up to iso800 after which the x100 blew the 5d away, no contest. obviously the 5d represents 'older' technology. from what ive seen, i think the mkii would probably beat out the x100 across the board. but it would also beat out the prior generation mki as well.
thus my conclusion that its not just sensor size, its the progression of technology. end of the day, the 5dmki was the 'pro' cam large sensor of choice for a good while. pro's used it to great effect across the publishing world. now its beat by an apsc sensor, which means that smaller sensor could also be used across the publishing world, at least in terms of IQ.
once you reach this level of pro result with smaller sensors, does then this argument make real life sense? if someone told you 5 years ago you could get the same or better results with a 'pocketable' cam as you got with your 5d that person wouldve been laughed out of the building! now that its come to pass i'm not sure the sensor size 'debate' is not over.