I've known about flow and experienced flow all of my life. I think I started to really understand it when I started programming in my early teens. I had no idea what it was called, but I definitely understood the absence of time, the stream of thought that just worked and the clarity of presence that accompanied it all. Kathy Sierra (her blog is suspended, but she is active on Twitter) talked a lot about the power of flow, and notably how so much on our computers and the Internet is a "flow killer" (i.e., Twitter).
Csikszentmihalyi is "the man" on flow, and has a book on the topic called Flow: The Psychology of Optimal Experience. I may need to read that.