The latest version of the beta of the Xbox One SmartGlass app hides an interesting detail in the form of the option to purchase a game directly from the app, as showcased in the picture below.

For the moment the option doesn't always appear, and clicking on it does nothing at all, but its presence seems to be a definite indication that the option to purchase (and maybe initiate the download?) games directly from your mobile device should be incoming, matching the feature already available on PlayStation platforms.

Of course we should stand by for an official announcement directly from Microsoft, but if you like to do your shopping on the go, this is quite promising.