Leaflet/Leaflet.label — pull request #27 by snkashis (2013-04-09 16:28:45+00:00)
Title: don't show labels for markers that have an opacity of 0.

People who may be using opacity to control visibility of their markers (referenced here https://github.com/Leaflet/Leaflet/issues/4) probably don't want any attached labels popping up for them during mouseover. This prevents that, with an option called watchOpacity.