Well, that was a short beta test. Amazon only first mentioned KF8 back in October, shortly before the Kindle Fire shipped. And I've only had my hands on the beta for just over a month. And now everyone has it.
(For more details on Kindle Format 8, click the 2 links above. )
Amazon is now going to let you submit an ebook with more complex formatting, but to be honest they've been doing that for over a year. Back in the fall of 2010, amazon distributed a new version of KindleGen that (whenever the source was Epub) started hiding the Epub in the back of the AZW file it made. At the time we all guess that they were planning ahead for something, and the Epub supporters were hopeful that Amazon might switch over to the better format. Well, we know that's not going to happen.
By all means, start learning how to use it. But when you do adopt it, it's going to mean an increase in some people's workloads in the short term.
Back when I converted ebooks for a living, I leveled out my Epubs at about the same level of formatting found in the Kindle format. Doing a second ebook for the more sophisticated Epub format would have added more to the cost than most authors wanted to pay (and I mainly did novels, which don't usually have complex formatting).
If you start using the new features found in KF8 now, you're going to have to also continue to make a less complex AZW file, because none of the apps or devices support the new features. If you don't then your ebooks will look bad to most of your readers.
In any case, here are the download links:
If you're in a hurry, just get the previewer. It can make a KF8 file from an Epub. And once you have the tools, please come tell me what you think. I want to hear your opinion.