William Fifer and his team from Columbia University recently used 26 sleeping infants with electrodes stuck to their scalps and faces to gain valuable data in support of the conclusion that babies learn while they are sleeping. Their data mainly consisted of increased brain wave activity occurring at the front of the babies' brains (the place most associated with memory) over a 30 minute span of listening to a musical piece.
I'm still waiting to read the actual research report and paper, but I can see the validity of their results if the research was done with high quality control. Infants motor and cognitive development often occur in relation to one another because they are in the sensorimotor stage of mind development. This when they learn through their senses (hearing, seeing, toching, etc.) which are still working while they are asleep.
Again, I need to read the actual study, but it gives further empirical support for parents to read to their babies, sing to their babies, and expose to lots of wonderful sensory experiences...even while they are snoring away.