Initially it was just a US roll out and then expanding to other English language search results, but now Rich Cards work everywhere worldwide.
The blog post was updated on March 28, 2017 saying “expanding Rich Cards globally.” The new portion of the blog post reads:
In 2016, we launched rich cards in the US, creating a new way for site owners to present previews of their content on the Search results page. Starting today, sites all over the world can now build rich cards across Google Search.
By building Rich Cards, you have a new opportunity to attract more engaged users to your page. Users can swipe through recipes in the UK from sites like BBC Good Food or browse movies in Mexico from Cinepapaya or view restaurants in Germany from Prinz.de. Also, rich cards support the AMP format. So if you build AMP pages, users will be able to swipe near instantly from page to page.
I did not find any Googler announce this but Shobhit Saxena spotted this and notified me on Twitter. He shared a screen shot of the updated blog post. I am a bit surprised Google didn’t announce it but maybe they will soon?
In short, Rich Cards are an expansion of Google’s rich snippets but add more richer details to the Google search results based on your markup. Here is a screen shot that explains it very quickly:
Forum discussion at Twitter.