The New York Times is reporting that Google may soon be launching an MP3 store - most likely in conjunction with a full release of Google Music (currently in Beta). If true, I would think it likely that Google would launch the service before Apple brings out iTunes Match later this month; hoping to gain some early traction before Apple stomps on them.
The main concern that would delay a release of an MP3 store is the record label deals which are necessary to obtain. Many record label frown upon music locker services (such as Google Music Beta) suggesting that they serve as a harvest for illegal, pirated, music. Without a doubt Google will be trying hard to regain the trust of the labels to help jump start their entry into the online Music industry - directly in competition with Apple, Spotify, Amazon and Grooveshark.