When powering a Hopper that is above a solid block or a container the items are held in place correctly within the Hopper. If however the Hopper is placed above a second Hopper then the second hopper will overpower the redstone signal and suck the items out regardless. This may not be a bug and be functioning as intended however, as Hoppers with containers above them are designed to suck out items.
Screenshot: Powering the 2nd Hopper in the chain as shown is completely ineffective as the 3rd Hopper still takes items out.
EDIT: This has been somewhat fixed in 13w02b by the fact that a powered Hopper now does not take items from above, so powering 2 sequential Hoppers in the chain would create an effective lock. Powering the first or the last Hopper in the chain would also be effective, this bug only occurs in long chains of vertical Hoppers, when trying to create a lock mid way down the stack.