I too have this issue from an upgraded 2.7.1 to 2.8 install, although mine seems worse as I tried even saving the post as draft then adding the image and it still does not create the gallery.
Tried in default theme and turned all plugins off, same result.
My theme is very dependent on timthumb grabbing an image from the post gallery and thumbing it for the main page… now I’m becoming imageless. The only one’s left are those posts created with 2.7.1…
Dev help would definitely be appreciated!
Cheers!