I'm unsure if this is the "correct" solution, but it worked for me in Maya 2011. I believe the problem lies in how you're saving out the image files. Make sure when you're saving the tiffs you save them on a transparent background (I did a crude cut out, but it's just an example lol) and with transparency checked in the tiff save options (at least for the image, not the alpha). When I reloaded the images in Maya, the projection worked fine.
I believe what's happening is that the incandescence is reading the white in the background of the image (not the alpha) as a value, hence making the cube glow. Honestly, I'm not all too sure if thats it, but it's my best guess at the moment. I'm going to look into this further, since it's raising a few questions for me as well and my knowledge of shaders could use some drastic improvement. Hopefully this solution works, let me know if it doesn't.