In an overview of implicit learning, cognitive psychologist Axel Cleeremans notes:
 

While it appears that awareness usually accompanies learning and might often enhance it, it remains uncertain whether awareness is always necessary for learning to occur.

(Cleeremans, 1998)

Read more in these two papers:

Cleeremans, A., Destrebecqz, A., & Boyer, M. (1998). Implicit learning: News from the front. Trends in Cognitive Sciences, 2, 406-416. PDF

Cleeremans, A. (1996). Principles for Implicit Learning. PDF