they;re not :glow in the dark: per-say. They have a protein that is excited by a particular wavelength of UV light (it's my understanding that they can be caused to glow at narrower or broader wavelength ranges depending on what a given research project requires, but this part's irrelevant) the protein, excited by the UV wavelength, emits its own wavelength of light that your eye perceives as green - red - whatever.
You would basically have to have a special light on them all the time to make them glowy, and I'm pretty sure that this kind of constant exposure will eventually cause bleaching that will be restored only by new cellular growth - or maybe that's just the GFP drosophila larvae that I was looking at.
Anyone want GFP fruit flies? I know where to get *those* - we could cross breed them to wingless flies and produce glowing wingless flies for people's dart frog tanks!