The wallpapers are available now for download via their e-store and cost $1.99. Carriers include AT&T Wireless, Cingular Wireless, Sprint PCS, and Verizon and their website has a list of supported cell phone models and manufacturers.
Right now, there are 28 Wallpapers available in four categories: New Magic Art (from Ravnica: City of Guilds), Kamigawa Block Art, Iconic Cards, and Core Set Favorites. But stay alert, because not only will we be adding more and more cards (including classic cards and Guildpact images -- before the set is even released!), we're constantly adding more and more cool features.