I noticed that when people look at the photos on their iPhones, they expect dragging to go to the next photo, so, this. It doesn't animate smoothly -- the new photo doesn't slide in, it just loads the new page with document.location -- but it's better than nothing, I guess.
The code is in dragnav.js. I think it might interact weirdly with pages that are zoomed in, but it's hard to tell, because the image-sizing stuff in gallery.js already seems to interact weirdly with that all on its own.
I've only tested this on iPad and iPhone. Does it work ok elsewhere?