Apple's Partnership with EMI to offer DRM free-music is likely to be the biggest move Apple has made towards seizing the market since the release of the iPod. Instead of playing the game, Apple has changed it completely, and now everybody has to either follow their lead or be left behind.
Microsoft loves DRM and proprietary formats, as it allows them to control what they do not own, a game that Apple has been playing with surprising success. However, with the availability of DRM free music in the AAC format, Apple has hamstringed other media formats. Any and all portable media players will be quick to offer support for AAC, because that gives them the possibility to play with the iTunes Store. Microsoft's own WMA format stands to be left in the dust, and now the Redmond Company will need to drop a massive investment into DRM development because DRMed music will quickly come to be regarded as plagued.
To make matters worse, Zune, Microsoft's own player, needs to inject DRM into the songs in order to take advantage of the main feature that differentiates it from the iPod, sharing. If AAC becomes the industry standard, player manufacturers will be ditching PlaysForSure in a heartbeat and like save a lot of money by not paying all those licenses.
Even if Apple does not sell one additional iPod because of this, if only half of the people posting online that they don't want to buy music from the Apple store because of DRM and the sound quality buy just one song, Apple's store would double their sales tomorrow. Furthermore, with the iTunes Store, they also put QuickTime on their computers if it was not already there, which means another loss for the WMV format.
All in all, this is a potentially huge shifting point in the industry, and the only chance Microsoft has not to be left too far behind Apple is to quickly ditch DRM themselves, which means walking away from the huge investment in it.