> You can in general never retroactively change a license, so their usage back then was certainly valid.
I would ask a lawyer about that. As it has been explained to me, the original author didn't have the right to distribute it under the MIT license, so they (rails) never had a valid license. It's similar with images, even when you grab it off flickr or another page and it specifies a license you like, that does not mean that whoever posted it there actually had the right to do that, and if they didn't, you can get sued.