It it possible to create a fluorescence shader in cycles?
From Wikipedia:
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
More specifically, Is it possible to make a shader that emits more light on one wavelength than it receives on that wavelength, while not emitting more total light than it receives?
Fluorescence is emission of light, so it would be natural to use emitter shader with the right rgb value mixed with other shaders -- it should give an accurate emulation of the phenomena. Please note that fluorescence includes time component, so the object might emit more light than it receives at the moment, if it was subjected to source of light earlier.
I think this is about the closest you can get with Cycles, but I'd love to be wrong. 
It uses the refraction shader mixed with diffusion to get the basic texture, and the glossy shader to get the actual "florescence" (and the color that it will fluoresce). 
The spheres and the plane at the bottom are all emitters, and you can see that the Suzanne mesh is a reddish hue where it catches light from the yellow sphere, and blue-purple where it catches the cyan sphere. This scene was rendered with 500 samples, no caustics, and blur glossy set to between 5 and 10. You can still see a bit of noise on Suzanne's head.
I think that adding the shaders to themselves should result in more light being "emitted" than received, but I'm not certain. If not, then there might not be a way to do that at present. It would be great if Blender had more ways to adjust the output from shader nodes, and more data on rays (besides length and type).
