I've just finished the first draft of a specification for Synchronization of Information Aggregators using Markup (SIAM) which is the result of a couple of weeks of discussion between myself and a number of others authors of news aggregators. From the introduction

A common problem for users of desktop information aggregators is that there is currently no way to synchronize the state of information aggregators used on different machines in the same way that can be done with email clients today. The most common occurence of this is a user that uses a information aggregator at home and at work or at school who'd like to keep the state of each aggregator synchronized independent of whether the same aggregator is used on both machines.

The purpose of this specification is to define an XML format that can be used to describe the state of a information aggregator which can then be used to synchronize another information aggregator instance to the same state. The "state" of information aggregator includes information such as which feeds are currently subscribed to by the user and which news items have been read by the user.

This specification assumes that a information aggregator is software that consumes an XML syndication feed in one of the following formats; ATOM, [RSS0.91], [RSS1.0] or [RSS2.0]. If more syndication formats gain prominence then this specification will be updated to take them into account.

This final draft owes a lot of its polish to comments from Luke Hutteman (author of SharpReader), Brent Simmons (author of NetNewsWire) and Kevin Hemenway aka Morbus Iff (author of AmphetaDesk ). There are no implementations out there yet although once enough feedback has been gathered about the current spec I'll definitely add this to RSS Bandit and deprecate the existing mechanisms for subscription harmonization.

Brent Simmons has a post which highlights some of the the various issues that came up in our discussions entitled The challenges of synching.