Incidentally, my daughter who is in high school and a budding photographer wanted to the fashion shoot thing with a friend. She is getting quite handy with a DSLR but usually sticks to auto everything mode. She asked my advice on how to get a "blurry background."
She wanted to concentrate on full length portraits, so I told her to take the K-x and M 50mm f/1.7. I showed her the diopter adjustment on the eyepiece and how to meter with the green button. I also told her to keep the aperture on f/1.7, focus carefully (with live view when ambient light allows it) and pose the aspiring model with the background as far away as possible. As I backup plan she took the Sigma 70-300mm along, in case the all manual mode didn't work out.
The nifty fifty did the trick. She posed the model very well, nailed the focus and exposure and got some splendid shots. Spectacular in fact, for a beginner. Especially when you consider that I paid about $50 for the camera and lens combined!
This thread probably helped me to give good advice. Thanks to all involved.