This is a common misconception. Ambient light KILLS contrast ratios.
Read:
http://www.projectorcentral.com/lcd_dlp_update6.htm
QUOTE:
"The consumer should bear in mind that while high contrast is critically important for high quality video presentation in a darkened environment, it is entirely irrelevant in commercial data presentation in a fully or partially lit room. Once you have lights on in the room, black levels get hammered, and the real contrast on the screen usually drops to well below 20:1 no matter what the theoretical contrast rating on the projector is. For "lights on" data presentations, adequate lumen output is essential, and contrast is meaningless."
This is especially notworthy if you have a good home theater projector or have calibrated top end projectors. When the lights come up, you really see how details are really lost. I think that people overstate the quality of a projector in a lit environment. If someone walks into Best Buy and sees the rear projection DLP units, then think they will get similar performance, in similar lighting conditions, with a H31 or SP4805, they will be very disappointed, and possibly angry about the quality.
HOW MUCH LIGHT IS IN THE ROOM ??? It really matters.