Jay Cross is well-known for his work on informal learning and I highly recommend his work. You can read his blog, here: http://internettime.com/?p=105.
He estimates that only 20% of work-related learning is formal and contends that school is a time-consuming and generally ineffective way to learn, and that most corporate training is wasted effort.
His definition of learning, which I really like, is: ‘Learning is any non-genetic adaptation one makes to interact more effectively with the ecosystems in which one participates.’
Jay reminds us of the importance of non-formal types of learning and I think a lot of what he says applies equally to language learning and teaching.