Also factors such as absorption rate can come into play. Adding fiber or milk to say 50-60 grams of protein may make it more likely to be absorbed more thoroghly over time rather than all at once.
It's generally agreed that the max is around 40 grams per sitting.
IMO, one can slowly increase the amount of protein per serving over a long time and have the body adapt to it, ie produce more proteases. Studies on omnivorous animals have shown that if the animal's dietary comp is slowly shifted towards a higher percentage of protien that they will produce more proteases in response. This of course is based on animals, not people and only addresses a normal maintenance diet not a mega-calorie high protein body building diet, but I do think it at least applies to getting yourself acclimated to higher levels of protein when your just starting to supplement.
The reason I say this is becaue I think we all remember our first experiences with the protein powder power flush..ie attempting to drink too much at once.