Wow, I'm really glad I found this site. Actually, I wish I had found it sooner, I'm sure it would have been a great resource for ideas.
I live in Phoenix, AZ and it's always been my dream to own an independent cinema. This may be closest I ever get, but I'm really happy with it.
I wanted to share my own backyard cinema project. I have to admit, part of the reason I settled on the house I bought was because of the movie potential in the back yard. The garage is detached, and the wall happens to be almost exactly in the 2.35:1 ratio.
The first screen I made was really cheap and simple. It was basically a large sheet of paper edged with duct tape. The idea at the time was to be able to remove the screen and store it in the garage. But, being paper, it wasn't sturdy enough to move very often and after a couple months started to crease and tear.
I investigated those inflatable screens but I wasn't too keen on spending over $600 on it, especially after I heard that the pump was continuous airflow and made noise.
So my friend said "What about drywall?" Seemed like a little bit of a risk because neither of us had done drywall before. And I knew that would make it hard to move. So we ended up with a permanent addition to the back yard.
What I like about the design is that it's capable of masking for 16x9 or 4x3 using panels that we slide in behind the frame. I'm really happy with the result.
Check out the pics:
This is the before pic. Kind of nasty looking, but can you see the potential?
And the finished result:This was built with 2x4s, drywall and paint. I doubt we spent more than $300.
16x9:
The best seat in the house. Needs some grass though.
Size reference:
Speaker setup. We have 5.1 surround. Rear speakers are on small tables and front speakers in front of the screen.

Custom built rack. We can adjust the shelves and go higher with the projector if needed.
From top to bottom that's 1) Projector 2) Playstation 3 (with Blu-Ray player) 3.) Receiver 4.) Amplifier 5.) Woofer
The projector.
Here's a flyer from the last event;

A link to the myspace page:
http://www.myspace.com/phxbackyardcinema