I'd actually started thinking to myself whether or not a medium armor that counts as light (such as Mithral breastplate) could be worn with the light armor proficiency, specifically while considering the revised Ranger. It was always a "Well, duh" in regards to the fact that virtual feats that only work in light armor would work in Mithral breastplate - I find it mildly insulting that Andy thought it needed clarifying.
However, I also decided that, if the armor typically is medium, that even if it counts as light armor, it still doesn't count for the light armor proficiency. After all, while Mithral breastplate may weigh less than steel, and be more flexible, thus allowing for greater freedom of movement, it still buckles up the same, still covers the same areas of the body, and is still breastplate. As such, knowing how to properly wear leather, or a chain shirt, or whatever, without stumbling about in it, or improperly putting it on, doesn't cover the typically medium armor breastplate, even when it's considered light.
I do agree it kind of sucks that a ranger now needs to buy the medium armor proficiency just to wear a light Mithral breastplate, but this is what happens when everyone unreasonably whines about how they should just give the rangers the feats without restriction, in light of the fact that they were trained in medium armors. However, outside of favored class XP penalties, picking up a level of fighter or the like shouldn't be too big a deal, even with the buffing up the ranger received.
