I think you can use more than 20g of protein, but that some recent studies have shown that more than 20g of protein doesn't contribute to muscle gain. I don't have time ATM to dig around and find it, but I know there was a Precision Nutrition blog post about it.
I don't buy it at all.
Can people honestly tell me that a 250lbs powerlifter or bodybuilder can 'absorb' the same amount of protein as, say, a 140lbs badminton player? It defies common sense...
Also, low carb diets become impossible going by the 20g per sitting logic, once you get above 3000 calories per day, unless you eat every hour.
Here's the PN blog post, I have it bookmarked
The main point, I think is that absorbing protein as new lean muscle tissue is not the only reason we eat protein.
For me, and i've said it before here I believe - I eat a sh*t load of protein and have done for years. Nothing 'bad' has happened yet. My toilet habbits are pretty much spot on, too, you know, encase anyone wonders. It's been this way for a few years now so, maybe after some more time i'll turn into the incredible hulk or something but, i'll take my chances...