This post made me look around last night and after reading a bunch of blogs and posts, I think $8 per person per night makes sense. Why? I saw where someone did the math. Take the Disney Magic for example, assuming 4 people in the room, $8 x 4 x 7 nights x 52 weeks x 44 concierge level rooms = $512,512 per year. If there are 7 folks on Disney Magic - 3 hosts & 4 bar / kitchen staff, that is $73,216 per person per year. And from what I read the hosts get a salary. $73,216 is just below the average US income for a family of 4 ($79,900).
It's likely close as guessing occupancy varies, concierge typically appears to be sold out, and some skip tipping while others tip well beyond $8 per person per day.
I'm with most in that we don't ask much of the concierge team. By the time we get on board everything is booked. Mostly will go get a drink or two each evening. One time one of the hosts heard me mention I enjoy biographies on Walt Disney and sent a DVD to our room. I tipped him extra for that special touch.