The Simple Sharing Extensions (SSE) from Microsoft is a specification that extends RSS allowing a bidirectional flow of data. I haven't read through the entire FAQ yet, but this sounds awesome! The SSE specifications are being licensed under the Creative Commons license, which allows anyone to use it and extend upon it as they see fit.
Microsoft's SSE FAQ states:For example, SSE could be used to share your work calendar with your spouse. If your calendar were published to an SSE feed, changes to your work calendar could be replicated to your spouse's calendar, and vice versa. As a result, your spouse could see your work schedule and add new appointments, such as a parent-teacher meeting at the school, or a doctor's appointment.
SSE allows you to replicate any set of independent items (for example, calendar entries, lists of contacts, list of favorites, blogrolls) using simple RSS semantics. If you can publish your data as an RSS feed, the simple addition of SSE will allow you to replicate your data to any other application that implements the SSE specification.
SSE FAQ: http://msdn.microsoft.com/xml/rss/ssefaq/
SSE specifications: http://msdn.microsoft.com/xml/rss/sse/