Saw a demo of a software application called Fluid Mask over the weekend. A company was selling it along with their backdrop image DVDs. Pretty impressive app.. I've only gotten my feet wet, but it can very quickly mask off a subject for insertion into another image having the background you want.
Here's a few expts I did today. The models are ones I've photographed. It also works as a plug-in for PS though I haven't tried that yet.