I know a lot of people will chastise me for saying the following, but it is what I truly believe, along with a lot of new research: stretching does nothing. It does not prevent injury, it could even induce injury if you stretch after a warm-up because you are just cooling your muscles back down and then warming them back up again. My roomate and I have done a "no-stretching" experiment, by working out the past few weeks (cardio as well as weights) and no stretching. The only exception is that I stretch each individual muscle after I resistance train it. I have not been injured or feel sore at all. I cannot truthfully say that I feel better than when I don't stretch, but this is far from an official scientific experiment.
It is true that stretching burns calories, but not nearly as many as doing actual resistance training, that could be done in the same time for 500% and up of the calorie burn, not to mention all the other great benefits of lifting. If stretching is the only thing you like to do, get a pilates video or something, they incorperate stretching with resistance training which I find a nice change of pace sometimes.