The New York Times has the answer today
Stretching: The Truth
If you’re like most of us, you were taught the importance of warm-up exercises back in grade school, and you’ve likely continued with pretty much the same routine ever since. Science, however, has moved on. Researchers now believe that some of the more entrenched elements of many athletes’ warm-up regimens are not only a waste of time but actually bad for you.
There is a good video there too (I've no idea how to embed that one in the blog....)