Finally! Amazon should have done this long ago. ePub v3 supports everything MOBI does and it has really extensive support across platforms. This is HUGE for people with a big eBook collection.
The most interesting thing here is that all of the 6 million articles on Wikipedia have been seen by 1 person at least once. So if we take all humans and ask them to search 1 unique thing on Wikipedia, would there be any articles that would get 0 hits?