Event when a sections' entry type changes layout?
Not right now, and technically Craft actually deletes the old Field Layout and creates a new one each time you save an Entry Type, since it would be more trouble than it’s worth to keep the old one and compare it to the new POST data for changes.

So, we could add a new onSaveEntryType event to SectionsService, which would end up getting called whenever the Field Layout changes. I’ll add that to the 2.3 list.