The issue isn't when it gets too real, but when it is almost real, but not real enough.

Avatar is the perfect example of getting real enough that the figures are believable. They still weren't entirely human, so our brains aren't as bothered when things aren't 100% right. Try to do the same thing with a completely animated human form and the subtle differences from reality will seem more pronounced. We know what real humans look like and how they move. The Avatar figures were close enough to human that they retained an emotional impact, but were just different enough not to trigger negative responses. Subtle differences from expectations were covered up by the alien appearance.

Polar Express and other animated efforts aren't trying to get full realism, but somewhere between realism and a cartoon affect.

I don't see actors ever becoming obsolete. Top paid actors earn a lot of money because they bring people to the theaters. Lesser known, but equally talented actors could just as easily do the movies for a fraction of the cost. It will always be cheaper to hire an unknown real person to do a movie than to do it with a computer character that isn't there. Computer voices are still way behind the state of the visuals, so real actors will be hired to do the voices. Most animated features stick to name actors even though they're never seen (which is always odd for the actors when they do movie press in countries where other actors are doing the native voices--a Johnny Depp will promote Rango in Japan even though it's not his voice the Japanese audience is seeing). Star Wars could not be made any cheaper with digital performers than with the then unknowns: Mark Hamill, Harrison Ford, and Carrie Fischer. The original Star Wars was made for less money than what it would cost to get Harrison Ford to do a sequel today.

Although many people rankle at the salaries that actors get, it doesn't bother me. When a Pirates movie makes half a billion dollars, should Johnny Depp get a nice big fat paycheck or the studio head that said "yes" to making the movie? I'd rather see the talent that made the movie get the money than the fat cats that arranged for the money. In the old studio days, that's how it worked. The big money went to the studios and not the performers.

Interestingly, if you look at the top 25 grossing movies of all time, relatively few had big name actors when they were released. The top grossing movie of all time, Avatar, starred unknowns--Sigouney Weaver only had a supporting role.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon