Do you want to own or rent your music?
Microsoft is developing technology called "Janus", according to this News.com article, that will allow customers to put subscription-based music onto their portable music players. Janus essentially is supposed to be a secure "stopwatch" that causes music to expire after the subscription period has ended.
Assuming that the software works and his hack-proof to the point of satisfying the RIAA and the subscription providers (like Napster), the focus then switches to the consumer -- does the consumer want to rent or own the music? Steve Jobs of Apple continues to maintain that consumers want to own their music, not rent it. The popularity of P2P and services like the iTunes Music Store seem to bear out this theory, but, to be fair, there is no real way to get subscription music onto portable players at this time.
What's in it for Microsoft? Well, it's rumored that MS is going to be opening its own music store in August and this software would allow them to reliably support both a purchase and a subscription option for their customers. Additionally, MS wants its digital rights management technology to become the defacto standard for equipment manufacturers, and adding this clock function for subscription service may certainly be a step in the right direction (you could certainly carry this forward to conceive of this type of protection for subscription-based video-on-demand services where the content might be loaded to the consumer's hard drive, but expire after the subscription period). Indeed some hardware manufacturers are already advertising their support for Janus, according to the article.