I found this when testing snapshot 13w05a in my redstone world.
I had built a few devices using a dropper/hopper based randomizer in the 13w04a snapshot, and they all worked as expected. After installing 13w05a I found none of them worked.
When an item is placed into a hopper by a dropper, the hopper does not change the state of any comparator attached to it.
1. Place a dropper anywhere.
2. Place a hopper facing the output side of the dropper.
3. Place a comparator for redstone output of the hopper.
4. Put any item of any amount in the dropper.
5. Power the dropper. A button works well for this example.
The dropper puts one item in the hopper.
The comparator would activate and stay on for as long as the item remains in the hopper.
The hopper puts the item back in the dropper.
The comparator did not activate.
The comparator becomes a BUD, activating by any block update around it, provided the hopper still holds items.