Either DRM or Interoperability
John Gruber just opened my eyes with an article detailing how we can have either DRM or interoperability. While he doesn't mention the case where some third party is paid solely for its DRM scheme by all other involved parties, he's generally spot-on: As long as each company has its own DRM, we won't get interoperability. If they share a DRM scheme, the owner of that DRM has a monopoly.
Unless there are some tangible and significant advantages to a particular DRM scheme, DRM mechanisms will converge on the one scheme that's used by the most convenient or cheapest stores. And what advantages could those be? Support for a particularly interesting platform is out, because interoperability implies all platforms support all schemes.
The only way to give us both would be to have some centralised conversion authority that you can ask to convert between the different flavours of DRMed files. But then how would one handle the mismatch in permitted uses? Apple lets you burn each playlist onto CD seven times (a popular workaround is burning a bought track on CD and re-importing it as an unencumbered MP3), others don't let you burn songs on CD at all, or a fixed number of times per song.
And that still wouldn't handle the case where one store only sells (and DRM-protects) music in a certain format, e.g. how Apple relies on AAC while others use WMV...
It's a complex problem, and it's definitely not solved by record labels publicly asking stores to open up while only licensing to stores that sell DRM-protected material.