Terry has an interesting post on how we learn to be leaders. I am inclined to believe that this is how we learn almost everything....not just how to lead! :-)
Reproducing an excerpt that I want to remember forever...
~ 70% of the learning is in the doing (learning by experience)
~ 20% of the learning is in relationship with coaches and mentors (learning from others)
~ 10% of the learning is in formal settings like seminars, classes, and training programs (learning in formal learning environments)
Now, I think there is a lesson for KM as well here. The tools for collaboration catering to the 20% learning and the personal KM tools catering to the 70% learning.
Another thing on my mind is that this distribution also depends the stage of learning that you are in. For example, when you begin to learn something for the first time, the training and learning from others would be more critical than at later stages. Another not-so-obvious example would be the need for refreshing theoretical concepts after a long stretch of practice that almost makes us forget why we do something the way we do it.