Microsoft announced last week that it is working with Netflix to roll its PlayReady DRM out to a wide range of consumer electronic devices that will play Netflix streaming content.  (PlayReady is already used on Windows and Mac versions of the Netflix app.)  Most of these devices are “TBD,” but one isn’t: the Apple iPad.

It seems that once you have a platform with APIs for app developers, you become a platform for DRMs — even if you’re a platform vendor that approves all applications and has its own proprietary DRM (FairPlay).

Several music services’ iPod/iTouch/iPad versions include content protection technology that is DRM by any other name.  These include the mobile versions of MOG All Access and Spotify Premium, which call it “file caching for offline listening” and/or “syncing files among your devices.”  (Spotify, for example, limits the number of files per device to 3,333 and number of devices per file to 3.)

The same goes for Android and BlackBerry apps, such as Spotify’s app for Android.

Netflix’s use of PlayReady is a form of streaming content protection that uses Microsoft’s own PIFF (Protected Interoperable File Format) — in the same way that Adobe Flash applications can use RTMPE for stream encryption and Flash Access DRM for downloads.  The music apps mentioned above use proprietary technologies.

E-book reader applications like the Barnes & Noble e-Reader and Kindle app for iPhone use DRMs on those platforms too: a variation of Adobe Content Server 4 and Amazon’s Mobipocket DRM respectively.

DRM for mobile devices poses some particular challenges in an age where popular application platforms are proliferating, as opposed to the age of the dominant PC.  I talked about one of these challenges two months ago, that of software hardening; other challenges will become apparent as the applications mature.