I have been going crazy trying to make my colorboxes load in an iframe, and just figured out my great stupidity.

The URL is built like this: [path]?width=500&height=500&iframe=true

Somewhere along the line I copied and pasted my path in such a way that it came out with the ampersands converted to &

I didn't notice. Now I have noticed. Changed them to plain ampersands and the iframe works. Sheesh. Hope this helps someone else.