At the New York Post Production Seminar last October, Douglas Spotted Eagle told us about chromakey screens from a place called EEFX. These screens are made from foam, are matte finish (not shiny), and EEFX sells remnants of the larger screens on Ebay! This 5.5' by 7' remnant cost Biddle $40 plus shipping.
This thing is portable, inexpensive, and versatile. Check out the video! All I did was roll it out on the behemoth table in Lab 136. It's not even properly lit (NOT recommended! Always light properly...unless you're really hungry and posting before lunch) A quick 3D Warp in Avid (it worked with the chromakeyer Final Cut Pro, too) and voila!
Store this thing inside one of those XXL Ziploc bags, and you've got a cool, portable green screen. Paint some clips green (Home Depot or Lowes) and you can hang it anywhere...just remember the basic rules for shooting a screen properly!