He started out by posting a link which contains the box-arts to every Xbox Live Arcade game yet; it sums up to around 600 titles. Then he posted a list of 36 unreleased titles, some of which are unannounced.
These also come with their respective box-arts which at the moment have not been officially shown by Microsoft. The unannounced titles include Double Dragon II, Star Wars: First Assault and Let’s Sing & Dance.
His post suggests that he obtained the information by snooping around on Xbox.com itself.