I'm curious, can you name an appropriate use case for a modal dialog? The first few uses cases I know of that came to my mind didn't turn out to be so great after I thought a bit about them.
I think there’s a usability concern only when the dialog presented is in some way dependent on the main page, e.g. if it provides some information that requires the original context. If you’re just showing zoomed-in photographs from a gallery, for example, I don’t see a problem.
