This bug reappeared again from 1.6.2/1.6.1
it's exactly the same as this bug technically https://mojang.atlassian.net/browse/MC-19886
guess this is technically a duplicate, why is this still a problem?

I first noticed the issue while playing SMP where I was making a rather large light farm like I usually do. Then I noticed that I couldn't get the wheat to "pop" off same with the potatoes.

So I tried numerous things to get it to work raising/lowering my redstone lamps, eventually I decided to test it with Single Player creative. To no avail I couldn't find anyway to get ANY form of light farm to work even using basic torches and removing them didn't work.

so it's a recent issue that I hope gets fixed soon.