first of all: I hope francesca, you agree to this share of knowledge and correct any “unsophisticated” statements… if u dont want it, just a word and I remove all the stuff from here. but some guyz (includeing me) asked for, so…
This is a basic setup. just to get the shader running. nothing about “color per vertex - cpv part” or anything else… its all fairly good described by francesca here in this thread.
here we go:
1) a basic scene includes at least
a) a dummy geometry (in my case the red box) - "carrier" for the ctrl_objectlights geometry shader
b) a light - in maya 8 an area light (gives a good result) - in my case a sphere area light. leave it at the origin. if u create more lights for that network - leave them all at the origin.
c) a geometry thats acutally our light source - I used a subD funky thing…
d) a backround geometry or what ever. u should c something that gets affected by the light
GIVE all those parts a proper name! (e.g. in outliner...)
[img]http://www.vrankovina.com/forum/basic.jpg[/img]
- Hypershade
go to the mental ray tab and look for these 2 shaders
a) mental ray lights -> ctrl_geocolor
b) geometry -> ctrl_objectslights
just click them (LMB) and u should find them again under the “Utilities” tab

- Attach the geometry shader to the “carrier” geometry… I called it dummy - as francesca did in the file included in the package
a) choose the dummy and open the attribute editor. go to the first tab, wher u’ll find the transform node name. (attribute editor opens with the “shape” node by default…)
b) go to the mentalray part and open the geometry shader.
c) ctrl_objectlights MMB and drag it from the hypershade over the geometry shader slot (the slot should get dashed frame) - release. u have connected the geometry shader to the dummy.

- Attach the light shader to the area light
a) choose the light and open the attribute editor. go to the second “shape” tab (opens by default)
b) go to the mentalray part, check “light shape” and give it some shape. I used sphere, but I guess it doesn’t really matter which one is chosen. for the samples value I just typed some nuber that gives me enough samples for sure - I’m certain, that can be optimized 
c) go to the custom shader part and open it. check “suppress all maya shaders (I guess this isnt a must…)”
d) MMB and drag the ctrl_geocolor shader over the light shader slot… - u have connected the light shader to the light.

- ctrl_objectlight setup
a) click the dummy. in the attribute editor go to the ctrl_objectlight tab. in the transform slot u shoud type the name of the transform node of your “light source” - u already know where u can find it 
b) in the light slot u should insert the name of the light SHAPE node (yourlightShape)

- ctrl_geocolor setup
this is the part where your fantasy should get wings… those are the main light controls…
just play with them…
soft light: color - well thats obvious. intensity - this is a co-player with the attenuation part… if u set it to linear or exponential or physical u should set here a fairly high value…
shadows - just check it out.
attenuation - if u know a basic light setup - this is all common
all other stuff is not an issue right now.

- set your render settings to preview and render…
u should get something like this

goooood luck
geometry objects, that should emit light do it as followed: