I recently came across an interesting feature of Stained Glass blocks: they have the ability to become illuminated or "lit up".

The square of Stained Glass on the left was placed after illumination technique was used to light up the right square.

My theory is that this has to do with how Stained Glass now occupies the space which Locked Chests used to take up and as most of you know, Locked Chests have a luminance level of 15 (the same level of intensity as blocks like Glowstone and Redstone Lamps). Because MCEdit thinks it's placing a bunch of Locked Chests, it lights up the area around them. Ingame, this lighting stays even though there's no block actually causing it.

There are probably a variety of ways to go about illuminating stained glass that I don't know about. However, the process I personally use is very simple:
1: Place your Stained Glass in a Minecraft world of your choice.
2: Open up the world using a version of MCedit released prior to the 1.7.2 update.
3: Edit the world in some way in the outdated MCedit program. It could be just deleting a single block, importing a schematic, etc. The edit doesn't even have to involve the Stained Glass
4: Save and close.

WARNING - This process results in the illumination of EVERY Stained Glass block in the world.

This feature is faulty and may stop working at times for reasons unknown.

Stained Glass placed after editing the world in MCedit will not have the illumination effect. The steps above can be reused/repeated if you want to illuminate more Stained Glass.
Small groups of stained glass tend to de-illuminate from time to time for reasons I'm not sure of. Placing anything ON TOP OF an illuminated Stained Glass block will also result in its de-illumination.

WARNING - This feature has not been confirmed by Mojang as official or purposeful and therefore has the potential to be fixed/removed in a future update. Build at your own risk.