Update - I also created a bug demonstration video available here: http://www.youtube.com/watch?v=eQlSnJtLUNc

Steps to reproduce:

1. Build a long redstone wire with repeaters far away from the spawn. ("long" = more than about 12 chunks; "far away" = teleport 1000 blocks away and you should be fine)
2. Go to the beginning of the wire and power it (by placing a redstone torch in front of it).
3. Follow the wire.

You'll notice something like in the screenshot "light-emission-bug.png": The light emission was correctly updated for all repeaters that have been located in loaded chunks when the wire got powered. (Like the right repeater in the screenshot.) It was not correctly updated for all repeaters further away. (Like the left repeater in the screenshot.)

This ticket relates to MC-1018. (Reason: If the lighting engine has to be rewritten anyway, this bug could be taken into consideration when designing the new version.)