As the price of entry-level home theater projectors drops below $1,000, the price of professional quality projection screens has become increasingly objectionable for new home theater enthusiasts. Nobody who spends $1,000 for a projector is interested in spending another $500 to $1,500 for a screen. You can always use a white wall, but there is no frame, and walls usually have texture that shows up in the image. Not only that but the color of the white paint on the wall is rarely conducive to giving you good color balance.  
Thus, the creative ingenuity of thousands of home theater folks has been unleashed to invent new ways to create screens at a fraction of the price of products available from the major screen manufacturers. Most of these do-it-yourself screens don't perform as well as professional screens, and the few that do can be labor intensive projects that require second and third attempts to get it right. But in the end, the huge benefit is that the DIY screens save lots of money...
Kassi42: sounds like something Greg would be interested in. I'll make sure he sees this link. 10.
Mac: You'd be better off using Screen Goo.  
Make sure to spray it, don't use a roller.
deathburger: I snagged a roll-up portable overhead projector screen from the local Goodwill for just a couple bucks.. it's more like 50 or 60 inch, but I certainly like the price.  
Now all I need is something that projects onto it... details, details. ;)
deathburger: Oh, and 10 & bookmark.
FoolProof: I once found one of those in a pile of stuff on the side of the road. The sign on the pile said, "FREE."
deathburger: Did you grab it?
deathburger: I've (now) got an extra display panel for a Stinkpad, gonna see if it's possible to marry it to an overhead projector somehow.
FoolProof: Hell yeah I did. It was in great shape. I don't even have a projector. (the one in the pile looked like crap. I doubt it worked within the last 20 years.