Open Media Library (OML), one of the most recent open source DVD library plug-ins for Vista Media Center has been under steady development and appears to be coming along nicely.  The majority of the new features and additions have been documented on Google Code, and a more in-depth overview of what they are working on has been posted at The Green Button. 

Some of the top feature additions and changes are posted below as well as a screenshot of the current UI (additional screenshots here).

Recent feature changes/additions

• MCML plug-in
• x86 and x64 support
• VIDEO_TS playback
• ISO image support with Daemon Tools
• UI redesign (pictured below)
• Generic installer done
• Command Line importer for My Movies XML files
• Speed Tests ongoing (20 Secs to load 8000+ titles)