With this post I'll introduce a new rating system: The Swampdrain Factor. A Swampdrain Factor of +5 means the particular development means the Swamp is being drained a bit through some innovation or best practice. A Swampdrain Factor of 0 means the Swamp is staying about the same level. And a Swamp Factor of -5 means the Swamp is rising through FUD or some innovation that doesn't lend itself to interoperability.
The Microsoft RSS calendar demo at Gnomedex has a Swampdrain Factor of +1. It opens interoperability a crack, provided Microsoft makes freely available the 200 lines of C# code they just showed that let Outlook users view an .ics calendar file, delivered as an RSS enclosure, side by side with an Outlook 2003 calendar. If this code doesn't become freely available, I'll lower the rating to a 0 until someone provides it in open-source form. (Microsoft releasing some of its RSS extension work under a Creative Commons license is a promising move.)
How would a technology get a Swampdrain Factor of 5? It would have to provide a true sync between two or more heterogenous calendars, automatically adding, moving, and changing appointments in a single calendar, presenting users with conflicts for resolutions, and all users to subscribe to these adds, moves and changes...using RSS or another syndication service.