As Dinnerbone tweeted, he changed the Hoppertimings in the latest Snapshot (https://twitter.com/Dinnerbone/status/307446745594478592).
After a short test I found out, that they now update every 9 gameticks (4.5 redstoneticks), instead of every 8 gameticks (4 redstoneticks), which makes it incredibly difficult to time hoppers.
For example if you want to get one item out of a hopper you now would have to depower it for exactly 4.5 redstoneticks wich is not really possible, or only possible with a pretty big circuit. You can't just depower it for 4 redstone because you would get 0 to 1 items and neither for 5, because you would get 1 to 2 items (It's unreliable).
Furthermore it doesn't make sense that a redstoneitem (like the hopper) updates every 4.5 ticks (wich is not an integer) instead of every 4 ticks (or another whole number)
After a look into the sourecode my assumption, that it's a bug got confirmed, because after every time, the hopper takes an item, it set's the cool-down to 8. The bug is that the algorithm checks if the cool-down is greater than zero. So eight ticks after the last update the cool-down will reach 0, but the hopper won't update until the next tick, wich makes it a total of 9 ticks in between 2 hopper updates instead of the wanted 8.