Google already has a place called the Google Solutions Marketplace, this is a spot for developers to place their third party apps that use Google API's.
However Google hasn't stated much about it but it sounds like the new marketplace will be very different.
App stores are all the rage these days, it seems. Now The Wall Street Journal is reporting that Google’s about to launch a new one for businesses, focused around creating software for Google Apps.
According to the report, Google’s plan is to sell software built by third party developers to enhance Google’s suite of apps (e.g. Gmail, Docs, Calendar, Sites, GTalk). These outside apps could be for adding more security, enhancing word-processing features, or porting information into Google Apps.