There are several services that can generate thumbnails like that, but most of them place watermarks on the images or place limits on how many API calls you can make in a month. (Amazon used to offer a paid service, but it was discontinued a year or two ago.)
To my surprise, when I was submitting an article to WPVote recently, I found that the thumbnails were served up by a subdomain of WordPress.com. I thought something along the lines of “Eh? I didn’t know they offered a screenshot service…” and continued about my business.
A couple days later, I spotted Ben’s blog post in my feed reader. In it he explained how he discovered a dynamic URL that Automattic uses, in places like the WordPress.org commercial themes page, to display thumbnails. He posted an email to Matt Mullenweg about it and got this as a response:
You can use it and link to it, but it’s not official. It’s not worth the effort to try to make it into a business – we have to support it anyway for our own apps.
Sounds good to me. Obviously it wouldn’t be nice to use it for something huge and high-traffic, but it seems like they don’t object to us smaller WordPress fanatics making use of it.
There’s now a WordPress plugin that makes it easy to use the thumbnails in blog posts, with a shortcode, or elsewhere with a template tag.