It was back on 03 Feb 2011 that WordPress.com made premium themes available to its users starting with Headlines from WooThemes and Shelf from The Theme Foundry.
Earlier today Michael Cain of Automattic announced the release of 3 more premium themes bringing the total to 41.

[...] week we mentioned that WordPress.com rolled out 3 new premium themes: The Great Adventure, The Columnist and Vintage [...]