Achievements earned in Xbox One games are now viewable on Xbox.com.
The change appears to have been implemented sometime yesterday. It also means the unlock pictures for Xbox One achievements are viewable at wallpaper size.
The most recent update to Xbox One's system introduced support for external hard drives and real name options in the friends list. July's update will deliver a feature called "achievement snap" which brings up a player's list of achievements in a side window as the game is being played.