In reply to Turdus torquatus:
The study sited is interesting and is sighted commonly, although it depends on your situation compared with those who took part in the study.
This is an interesting study suggesting that (for muscle building aspect only!) that 20g is the maximum absorbed.
http://ajcn.nutrition.org/content/89/1/161
However, for you're average trainee then yo're looking at around 120g of perotein per day, and assuming eating around 3-4 times a day then it's 30g. Higher protein diets will leave you feeling fuller and displace the absorption of other macros so may be beneficial.
It all depends on your goals, if you're trying to build muscle then a higher protein amount may be required. It will depend how many calories you need to consume, how many times a day you want to eat and the type of training you are doing. Endurance based training requires a lower amount of protein (circa 90-100g). What is your gut uptake like. Are you taking a whole source of protein, or BCAAs, etc etc.
It's all up to you really, I wasn't trying to be too prescriptive - see what works for you. All in all it's a complex chain of systems, and many factors will play a role, so I think it's difficult to pin point a specific value. Interesting subject though for sure!