I'm glad you all like the idea! After looking at facebook, it looks like there are a couple ways to do this:
1. Like facebook: the photo description in the gallery is one and the same as the photo caption in a photo post. If you change one, the other changes too.
2. Alternatively, when you post a photo, the photo description in the gallery would be copied into the post as a caption. The caption text would be just like any other text in the post. After that, if you change the text in the post or the gallery it would not affect the other.
Either option would be great, but I had in mind the second. Exposing my geekyness a bit, the second should be pretty easy build because of the way the system uses markup based posts. The first option will probably be a PIA. I'm lazy, so I was thinking of the first option.
If you go with the second option, the check box label should probably be something more like "Copy photo description to caption in post" rather than the wording above.
The end result would look something like these posts in that same thread:
You guys figure out where the captions go... above or below the photos. Traditionally they are below. In those last posts they are above. I've done them both ways. If you build this feature they will at least be standardized, adding some consistency that should make for easier reading.