Projectionists dimming projector bulb wattage in order to extend the life of the bulb is one issue. However, much of the problem of late, stems from projectionists refusing to change out 3-D projector lenses when not projecting a 3-D film. This results in a dark, muddy 2-D picture. The process of changing the lens is long, and sometimes complicated. So, many projectionists simply don't bother.
I was pleasantly surprised to read that less than half of Pirates of Carribean's opening weekend gross came from 3-D showings. The darker image projected for 3-D films is sort of a buzz right now. If it grows, this could be the beginning of the end for this go around of the 3-D trend.
At the end of Ebert's article, he quotes from Matthew Humpries at Geek.com :
What can you personally do to be sure you see an ideal picture?
• The title of the movie listed by the theater will have a "D" after it if it is being shown on a digital projector
• If you are in a D movie, look at the projector window when seated. If you see two stacked beams of light it is a Sony projector with the 3D lens still on.
• A single beam of light means no 3D lens, or a different make of projector that doesn't have the issue
• If you see the two beams, then get up and go complain. You paid good money to see the movie, so make a fuss until they either give you back that money or remove the lens. Seeing as that's an involved and time-consuming process, expect a refund.