Of course, DRM is already being used in a number of subscription-based music services, and even Apple's iTunes Music Store uses the technology to restrict how many PCs a downloaded track can be played back on. In a PC environment, it's easy to restrict access to a track, but portable devices don't yet have the hardware or software to effectively enforce DRM restrictions. Microsoft is out to change all that and is working with manufacturers to integrate hardware clocks into portable devices so that the devices know when a user's subscription to a track runs out.
Fortunately for Apple, the iTunes Music Store involves track purchases rather than subscription fees so there's less of a need to deal with DRM on portable devices. The iTunes Music Store lets users copy tracks to an unlimited number of iPods and burn tracks to an unlimited number of CDs, but that sort of freedom wouldn't fly with a subscription-based service where users don't actually own the content they're listening to.
It looks like the battle for dominance in the online music business may come down to a brawl between Apple's iTunes Music Store and a slew of subscription-based services backed by Microsoft's DRM technology. Honestly, I'm not sure which system has the edge, since song ownership and subscription services each have their own benefits. One thing seems certain, though. Until these services become a lot cheaper, Kazaa will continue to be the most popular download on the Internet.