Whether you're doing portraits or at times shooting green screen video on location, a quick portable backdrop system is a convenient tool to keep around. Here's a look at the PXB Portable Background System from Studio Assets that uses a patented method of framing up an 8x8 muslin backdrop.
As mentioned, it requires special 8x8 muslin backdrops that have pocketed corners for the rods to fit and are available in the most common colors. Of course I can't imagine that it would be difficult to sew up your own if you really had to. You can find the PXB Portable Background System over at SeaPortDigital.com (found here).