According to the wikipedia page at http://en.wikipedia.org/wiki/Mean_squared_error, it has:
Note that, although the MSE is not an unbiased estimator of the error variance, it is consistent, given the consistency of the predictor.
However, I saw from literature that MSE is indeed an unbiased estimator to the variance. My textbook does say it's an unbiased estimator. Is the wikipedia correct?