Thanks for all your help, guys! Unfortunately, I wound up with a client who doesn't really understand refraction and kept trying to have me remove "the dark parts" until it just looked like a piece of glass. After that, she had me light it from the front only to get really hot highlights, ruining the glow from backlighting. Then she didn't like that it no longer looked soft.
In the end, I even had to switch back to Mental Ray to make the deadline with an outsourced render farm. :\
Still, I appreciate the feedback and support! I'm going to keep an eye on Corona and keep playing with it when I can. It's really great. :D