I don't think bukkit was confirmed; I've merely suggested that it -could- be an issue (thus logically if it's in use we should be told). However yes, aside from direct path from tile to sky interrupted by NOTHING except glass would be the next step to confirm.
We're using the in-built function of MC to determine whether a block "canSeeTheSky". It's the same function used to determine whether a block will have a light level of 15 during day.