I went back and looked at my photo album from last year’s vacation, and remembered there was one building I failed to identify:
http://img83.imageshack.us/img83/8412/img2724aa9.jpg
I remember it as being pretty close to the Colosseum, but I don’t think it was part of the Forum. If nobody recognizes it instantly, I’ll figure out what roads I was on.
Here’s another picture looking at some other buildings from within that structure.