The idea of being happy by being in the present or in the now was made popular by Eckhart Tolle with help from Oprah Winfrey. But that is not where it began and it does not end there. For example Oprah Winfrey does TM (Transcendental Meditation) and encourages others to do it.
Buddha said something about this and Eckhart Tolle tells many Buddhist stories in his latest book, A New Earth. Also a brilliant psychologist, Abraham Maslow (1906 to 1970) said "I can feel guilty about the past, apprehensive about the future, but only in the present can I act. The ability to be in the present moment is a major component of mental wellness."
There is also a new psychology called Positive Psychology that studies how people can be happier and knew about this phenomena before Tolle wrote his first book. They call it the flow (like being in the moment or in the zone). So this article covers all this and more. Here is a quote from Tolle: "You find peace not by rearranging the circumstances of your life, but by realizing who you are at the deepest level." Is the Key to Happiness Being in the Present Moment.