If you have an object which the opacity slider is not set to 100% opaque you can NOT use it as an emissive object for GI.
You can demonstrate this to yourself by turning off IBL and all lights in a scene, turning on GI, and putting a cube on top of a plane. Set the material on the cube self-illumination to some amount and you'll see it glow AND light up the scene. Now adjust the opacity slider even a tiny bit -- once you go down below 100% the GI stops working.
There IS a workaround -- you can put an opacity map in the opacity slot and this will correctly affect the transparency of the object will still allowing it to do GI. However, this takes up your opacity slot (which you may be using for other things, like controlling what parts of the object are completely visible). It's particularly annoying for a Popvideo (which takes other that opacity slot).
Because the Opacity map does work correctly this shows me this is indeed a bug -- you ought to be able to adjust the opacity slider and still have it emit light.