Mike recently submitted a great question via email:

I’ve been reading about the new iTunes Store selling unlocked music.  Will this music play on my Touch Diamond?

Yes it will play on a Touch Diamond, or any Windows Mobile device for that matter. 

Apple has actually been selling unlocked, or DRM-free, music for many months, but the unlocked tracks were available from a few specific music labels.  Apple recently announced that it will sell only DRM-free music by April 2009; in the meantime, the company has removed DRM protection from about 80% of its current offerings.

These unlocked tracks are encoded in the AAC format which will play in Windows Media Player 10 Mobile (the default WinMo media player) and in most popular third-party Windows Mobile media players.

You can tell whether or not a particular song sold in iTunes is unlocked by looking for the plus icon in the track information line:

When iTunes is 100% DRM-free, this icon will likely be retired, but for now it’s the only way to tell if a song is unlocked. 

These tracks are DRM-free and can play on any AAC-ready device, not just Apple’s iPods or iPhones.