JCambert,
There are certainly cases when lines wind up being over or under-estimated, but I think that's primarily due to studio projections being off as well. I'm fairly confident oddsmakers get the tracking numbers and then shift the lines based on the action they get. I stopped betting these props a few months ago, but I still keep up with them, and there have been many instances where the gross fell into just a few hundred thousand dollars of the O/U.
A good example is the McGuire movie...that's cutting it real close!
Just for grins, let's archive the current numbers for this weekend and see how close the actual #s turn out to be...
From OLY
Daddy Day Care: O/U 26 million
From WSEX
DDC: U/O 25 with the Over at -190.
From these lines, I'm going to guess that the movie comes in at 26 mil and some change. Problem is, a couple hundred grand would make all the difference. If the lines were regularly off by millions, I'd be more willing to play. But it seems like it's always a pretty close contest.