If a hopper is receiving a redstone signal, it will, as it is supposed to, not send any items in it to whatever block with an inventory it is pointing at. But, at the same time, it won't pick up any blocks dropped or thrown on top of it, or suck any items out from any block above them. The only way to introduce another item to the hopper is by having another hopper feed the items into it (so the items are getting "pushed into" the hopper, rather than the hopper "sucking them out").
I'm pretty sure this is a bug, as hoppers were able to pick up/suck out items in earlier snapshots, even though it was being powered.