The Observer will detect if a Copper Bulb is powered vs. if it is actually emitting light or not.

The Observer does not detect if any other block is powered (i.e. stone, wool, wood), and should not with the Bulb. More related, it does not detect if a piston is powered and unable to extend (i.e. is trying to push Obsidian), but will detect once extended. 

The Observer and Copper Bulb should behave the same as other blocks in this instance.