when i was a kid i used to go to sleep with the radio on. in the country town i was from all we had at the time was an AM station that played classics.
as a teenager i found that i knew the words to _heaps_ of classic tracks. i'm not sure if its because of the conscious or the unconscious (ie sleeping) (or a combination of both) listening i did, but found it weird.
remembering this (in my 30's now), i thought it would be and interesting experiment to try it again, pretty much trying to pick stuff up by osmosis.
I have some learn french audio books and put them on in my ears while performing various tasks. the idea is that i dont concentrate on the book per se, but the other task at hand.
for me in my 30's now, there wasnt the expected "kung fu, i know kung fu" moment, so i thought, as i dont use french in my normal day to day activities it didnt really work so well.
later on (with my french speaking GF) we would have a chat and i would ask things like "is this a french word" etc etc... i'm not sure these words were from previous conversations, or whatever, so from a scientific POV, its pretty much a negative... in my unscientific opinion however, i think that i have absorbed a bit of french from the osmosis type of learning.
i guess it depends on your goals and what you want to achieve.
if you want to pick up hard facts quickly, then learning by osmosis, at least in my experience, is not a very efficient way of doing it.
if you just want to pick things up "along the way" the learning my osmosis may work for you, however keep in mind you may not have the "kung fu, i know kung fu" moments.
as always YMMV :-)