We do know that climate is always changing, but a very varying rates. We do, through ice samples have good data about how the climate evolved over millenia. And we know quite conclusively, that man's activities are warming the planet rapidly due to the levels of carbon released in the atmosphere. There's no controversary in what I've just said, unless you believe the President or same fringe scientists. But we don't have data that we could use for weather forecasting/modelling for sure.
What we don't know is exactly how that will affect real weather that you and I are interested in. We don't know how it is affecting modelling. In that way, Scott's point is a good one, and one that I hope scientist and mets start to look at closely... I wonder if there is enough data yet to say that the modeling is really being affected? It could be that the improvement in computing power is temporarily offsetting the loss of an ability for models to predict based on past behavior over the last 100 years. Who knows.