While most subscription music services allow one to download an unlimited number of tracks for a flat-rate fee, the current main catches include losing all tracks downloaded as part of the subscription once it expires and requiring a compatible player to play them on.  For example, music bought on Microsoft's Zune service will not play on Windows Media PlaysForSure DRM compatible players or vice versa and no unlimited download subscription music service has so far been compatible with the iPod.

Now Sony BMG Music Entertainment's CEO has mentioned in an interview with the German national newspaper FAZ that the company is working a subscription model that will offer compatibility with all portable music players including the iPod, going by this Associated Press article.   Sony is proposing a subscription fee of around €6 to €8 ($9 to $12) per month for unlimited access to its entire catalogue and that songs would not expire once the subscription has expired. 

So far a Bertelsmann spokesman has confirmed the accuracy of the interview, but would not provide any further info.  No info is available yet on when this service will go ahead, however, going by the interview, there were talks being made with other major music labels also. 

In my opinion, if all the major labels jointly offered such a subscription service, particularly along with a mobile operator, this would not just be an iTunes killer (unless Apple actually takes on such a service), but also a major killer to the Audio CD format, not to mention a potential final end to high-street music stores.