Not sure if this bug/glitch is fixed yet, or whether it is meant to happen.
Basically, when you trigger a redstone current towards a piston, the piston emits light.
To recreate:
Watch my video: http://youtu.be/S3V4rS_ddcQ
OR
1. Dig Hole 2 Blocks deep, 4 Blocks across.
2. Place piston at the furthest block.
3. Put redstone repeater next to piston.
4. Run a redstone trail from repeater to the end of the trench.
5. Cover all up except the piston.
6. Place a lever/ pressure plate on the block covering the redstone trail (the furthest one away from the repeater)
7. Flick lever/ Step on pressure plate.
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Thats not the piston emmiting light, its the repeater! Currently pistons let light pass through them, but this behavior is going to change with 1.5
I think this may have something to do with the fact that the piston arm is non-solid (and transparent) and perhaps the main piece of the piston becomes non-solid when it is extended? Anyways, the redstone repeater is probably the cause of the light. If you power the piston with a redstone torch under a block which is under a piston and remove the repeater, does the light level go down? The "bl" section of the debug console will tell you if light is actually getting through or if it is a rendering glitch.