I don't know if you've checked this out yet, but there is a free program called Google SketchUp that is starting to make its way into the repertoire of some big-name artists (Joe Quesada, Stuart Immonen). There are a ton of free libraries available from Google as well that you can use to begin to set up rooms, streets, and other locations for use in your background work.http://www.google.com/sketchup/index.html
I gave this a "dry run" with a test page. I set up an apartment, took some screenshots of "camera shots" I wanted to use, and lightboxed these guides onto my art board. I then added in details, texture, and shadowing as I saw fit. Here is a shot of the apartment model and the page I used it for:
It's a nice tool to work with for creating backgrounds, especially interior scenes. I suggest you all give it a look -- the price is certainly right!
(By the way, here's a link to Stuart Immonen discussing some of his referencing techniques, including SketchUp: http://goodcomics.comicbookresources.com/2007/04/03/stuart-immonen-on-computers-and-art/