There's another post on this as well - Biome Detail - Bioluminescence But yes, I'm very much hoping that this makes an appearance. Little details like this are what can make a game so much better. I would also like bioluminescent plants to be ground up and used to create both continually bioluminescent and glow-in-the dark paint pigments, hair dyes, etc... It would be very neat to be able to write on walls and only have it visible once the lights are shut off.
I always had a buzz when I found the 'blue mushies' as I'd like to call them, and if an underground biome to be implemented, generate the same feel of achievement and determination. It would also have to be unique and distinct from it's surroundings.
A planet with permanent night? They could do so much with this! And having a pet that would illuminate dark places would definelty be a plus.