[devilsgrin]

i always thought that the diminishing of the Jedi's ability to use the force that Yoda mentions was in relation to (specifically Yoda's him being the most powerful jedi) sensing movements, happenings and even eventualities - basically prescience, throughout the galaxy. It would reason that only the most intense uses of the force (or emotions of force users) would be detectable - even by Yoda - as seen in our green friend sensing Anakin slaughtering the Sandpeople.
It seemed in Ep III that he sensed the mass deaths of the Jedi after "66" is initiated. The deaths of the more powerful Jedi especially would have been sensed certainly.

The reason i believe this restriction on Jedi powers was caused is not because the Jedi are unwittingly aiding the Dark Side... but that the Force itself senses the ascendancy of the 'Sith' and as the Force is neither light or dark in itself, these concepts exist solely in the minds of others, it would seemingly be tuning into the dominant (or at least soon to be) Force users in the galaxy. the force is life afterall... and life is emotion. Jedi deny emotion, especially the strong ones, the Sith are the opposite, so their desires would influence the Force more strongly. Since the sith are secretive and sublte, a 'cloud' of secrecy could conceivably be distorting and blocking the senses of force users more attuned to a Jedi mind-set.