New (Grey?) Screen Or New PJ For Better Contrast/Blacks?

You are correct in your thinking about the ceiling and walls. In your case the ceiling is in need of some treatment to eliminate or reduce the reflected light.
An inexpensive way to test this is to buy a single black flat sheet (Bed bath and beyond) about $10 and attache it to the ceiling in front of the screen with some duct tape or whatever mean you can figure out, depending on the type of ceiling.
If you image improves greatly the paint the ceiling mouse ears black or attach triple black velvet (best light absorber there is). you will probably only need to treat the ceiling 5'-6' out from the screen.

Another thing that will effect your black level is having the projector properly calibrated. An incorrect brightness setting can really reduce your blacks. If you have not used one of the consumer calibration disks to properly adjust the user level setting I would suggest doing so. There is a free BR disk you burn to standard DVD with a standard DVD burning found here http://www.avsforum.com/avs-vb/showthread.php?t=948496 or you can buy the Disney WOW disk which is geared to be user friendly.

I would do both of these steps before thinking about replacing your screen.
If you are in/near the Philadelphia PA area, drop me a PM, I am a DIY Calibrator and work for experience and tips.

At least the sheet would let you test it out without any permanent changes and if it makes a big difference, she will be able to see it as well...

You have a very bright projector and most likely are seeing reflected projector light coming back to the screen off the ceiling and other surfaces in the room. A bright projector in a less than perfect setting will most likely want a gray screen and or some treatments to that ceiling to improve CR. Projectors do not project black what they try and do is project nothing and what you see as black is what the screen looks like to your eyes under whatever light is in the room lighting up that area of the screen. Likewise for all dark colors and shades of black. If you have done a calibration on brightness then your next step would be to try a darker screen. Get a piece of anything that's a medium shade of gray and put it up to the screen as a test. Gray paper for instance with a flat finish nothing glossy, and see what you think.

What you are seeing as better contrast during bright movies and poor contrast during dark movies is a good example of perception of contrast. I answered a similar question a few posts ago on this same page.

How about shadow boxing the screen? How far out from the screen do you think you need to project the shadow box to improve contrast? This will noticeably darken the walls and ceiling due to reflected light off of the screen.

As a test you can hold a panel along the top or side of the screen to see the effect on the adjacent surface.