I've come to see that you can do more and more with just one speedlight. I just watched the OneLight Workshop by Zack Arias, and it is wonderful what one light can do. I am still developing how to shoot with off-camera flashes, and am understanding that when shooting portraits, the one main light is key, and additional lights would add on to enhance the shot, e.g. kicker light, hair light, fill light, etc.
Well, here is another video with Joe McNally talking about some of the shots he made with only one speedlight, namely the Nikon SB-800.