With the RC version of Web Playlists, we moved from a custom XML format to a SMIL-based format for storing playlists. As a result of this change the old isx format has been retired. However, Brian Blum (a dev) in our team crafted a tool to help convert the old files. The tool can be downloaded here. Thank you Brian!
I am pleased to announce that the Web Playlists Release Candidate (RC) release is ready for you to download and deploy.