If the intent is to use the projection system full time without full light control, or at least significant light control, then forget it. Period.
If the goal is to add a really large display (110"+) into the room as an added feature to the 70" display you already own, then there are some definite options out there.
1. Use after dark! Once the sun is down, there is nothing which holds the room back from being a true theater. Perhaps the paint is a bit lighter than it should be, but 110"+ projection can still look great in this environment.
2. ZONE YOUR LIGHTS! - Don't just put up 16 in-ceiling cans and put them on one switch! Instead, turn on the brain and think about the layout of the room and where you need light to be after dark. The place you do NOT want light is near the screen, so make sure every single light near the screen can be 100% eliminated. Then, you want lighting over the seating, preferably the furthest from the screen as possible. Finally, you want controlled lighting. Not a lamp or chandalier, but a honestly directional light with a 30 to 50 degree field of lighting. Ceiling cans with halogen spots can do this nicely. Often more is better than less as you can add a ton of light without dramatically impacting the screen.
3. Get some top shelf blackout blinds. We aren't talking curtains here, but some real aluminum backed, completely opaque blackout shades which are designed to make a bright room dark when wanted. If you have mulitple windows, consider dual blinds which can reduce light at 100% and 50% depending on your choice. You don't have to close all of them if you want some daytime viewing, but you will then have the option.
4. Rear projection (as mentioned) using StarGlas from Stewart has been used in poolhouses to allow people in the pool to see rear projection during the daytime. If you have the environment which allows for it (very rare) it can give you the size and image with very little light control necessary.
Take a look at:
http://www.avintegrated.com/lighting.html
5. A really bright projector! - Yes, they are out there. Runco has them (Blech!) as does Digital Projection (YAY!) and they can overcome some significant shortcomings in a room.
6. Set your expectations: The bottom line is that whatever shade of black, grey, or WHITE your screen is, that is the darkest color you can achieve in the room. Our eyes are really amazing at playing tricks of light and dark on them, but a very bright room simply won't allow for anything near a true black to be on screen.
