Core strengthening has become very popular. Unfortunately, it may be causing greater imbalance in the overall tension and tensegrity of a persons body. The idea is not a bad one as long as the core is integrated with the rest of the tensional structures that make up a body. Stretching and releasing the core (flexibility) is even more important when strengthening the core is undertaken.
The following blog post by Walt Fritz, PT on the massage and bodywork professionals website makes some interesting points to support his issue with core strengthening. It is short and worth reading.