How often should transmission fluid be replaced?

With today's virtually sealed automatic transmissions, the auto industry recommends that your automatic transmission fluid be changed at between 75,000 and 100,000 miles. In some cases, GM, for example, their literature maintains that their Dexron III automatic tranny fluid can run up to 150,000 miles before needing a change. So, how often should you have the transmission fluid replaced? A good rule of thumb is to have your car's transmission fluid replaced at 50,000 to 75,000 miles, not forgetting to replace the automatic transmission fluid filter.