I've decided to leave the BlogLines and Newsgator synchromization APIs alone - the former is incomplete, and the latter assumes that the definitive copy of the data is on the server side - they use their own ID for each item, and expect you to synch with that. Thanks, but it sounds like that's more trouble than it's worth.
So it sounds interesting to me that Google is jumping into the fray:
Google plans to offer a feed reader API to allow third-party developers to build new views of feed data on top of Google's backend. The new APIs will include synchronization, feed-level and item-level tagging, per-item read and unread status, as well as rich media enclosure and metadata handling. Google Reader PM Jason Shellen and engineer Chris Wetherell both confirmed Google's plans after I posted my reverse-engineering analysis of the Google Reader backend.
With any luck, it won't suck :)