Today I was playing with Gmail's IMAP implementation and Outlook 2007/Pocket Outlook. As a result (not counting a couple of known issues that will hopefully be fixed soon) I can read email in native clients on my work PC/laptop, PDA/Phone and use Web UI elsewhere (like wife's PC, daughter's PC, mistress's Mac ;). And all my read/unread/deleted mail will be in sync without additional efforts on my side (fingers crossed).
My next thought was: I want the same for my RSS! I used RSS Bandit back in the day when online RSS readers sucked. But synchronizing read/unread items between work and home PCs was pain in the ass and I didn't even read my feeds on my phone back then. So I moved to Bloglines, then to Google Reader, now back to Bloglines (beta). But it would be super cool if I could use desktop RSS reader when on my own PC and web-based reader when elsewhere and this wouldn't require jumping over my head to keep all the feeds and read/unread items in sync.
Creating such a protocol (let's call it IFAP) is not a very difficult task. The difficult part is making it de-facto standard so that both online and offline readers support it and writing/hosting IFAP servers. And this is a task that only someone like Google or Microsoft can pull. Or probably some very ambitious and aggressive startup. Not me, unfortunately.
So, if you have balls of steel (or just happen to work for Google) and you like the idea, please, oh please, implement it. You can count me in as your alpha-tester.