Ben has written that Songbird 0.2.5 will be able to play DRM-protected tracks. To quote him:

The next version of Songbird (0.2.5) will support Apple FairPlay and Windows Media DRM audio playback. Those features are already enabled in the latest nightly build and everyone is encouraged to give those builds a spin. If you're on Windows you'll need to have a new-ish version of Windows Media Player (probably 9 or newer) for protected WMA playback and QuickTime for Windows (probably 7 or newer) for FairPlay to work. On OS X you'll only get FairPlay playback, sorry.
Running the DRM Guantlet | Songbirdnest.com