I have been experimenting with irridescent like effects in cycles for some time now - and came up with what I believe is a novel approach.

The way this works is to essentially subtract one glossy shader from another by feeding negative RGB values into one of them. This has the effect of forming a glossy halo around any lightsource hitting the object.

By overlaying different coloured versions of this subtracted glossy with different roughness values - you get a rainbow halo effect around glossy reflections.

This trick also works with the anisotripic shader as shown in one of the images (simply replace the glossy nodes with anisotrpoic nodes)