@eliobasagni, most free themes at WordPress.com are downloadable from the sidebar on pages like http://wordpress.com/themes/illustratr
However, Illustratr doesn't have a download link yet because it requires some special code support for Custom Post Types that will be included in an upcoming Jetpack plugin release. Once that's released, then the Illustratr theme will get a download link added. You can watch http://jetpack.me/news/ for updates—there's a subscribe link at the bottom left of the page if you want to get updates by email.
Glad you like Illustratr. :) Thanks for asking about it!