With no items in the hopper I walked over 300 blocks away from the comparator and into a nether portal. I returned and put 28 blocks into the hopper, more than enough to send power to the comparator, but it did not output a signal(image 2013-01-29_14.04.11.png). I then broke the netherrack next to the comparator and the signal was output(image 2013-01-29_14.04.26.png)

Problem also persists when the signal is at 1 and you put enough items in the hopper to increase the signal to 2. In conclusion, the comparator will not update its signal output if the chunk it is in becomes unloaded and the reloaded again unless a block update happens.

Was playing on a multiplayer server