When a Redstone comparator is used to output a signal from an item frame, the following is supposed to occur:
Empty Frame = No Signal
Pos. 1 = Comparator Lights Up, 1 Block Signal
Pos. 2 = 2 Block Signal
...and so on.

I have noticed that although there is no signal from an empty item frame, there is also no signal from position 1. Further, position 2 gives only a 1 block signal, position 3 gives a 2 block signal, and so on.

To reproduce the bug:
1) Place an item frame onto a block and have a Redstone comparator drawing power from the back of the block.
2) Fill the item frame with an item, and it should not give a signal [BUG]. Turning the item within the item frame will strengthen the signal up to 7 blocks [should be 8], until it returns to position 1, and no longer gives a signal.
3)This can best be shown by placing 8 Redstone leading from the comparator and placing a Redstone lamp. As the comparator can only reach up to 7 Redstone dust away [BUG], the lamp will never turn on.