A little while ago I noticed the Mozilla Raindrop project. It's an interesting project in Mozilla Labs by the Thunderbird team.
It's trying to rethink how messaging (in general, not just email) should work - using open web technologies. On the surface it sounds like Google Wave, but I'd say it's aiming a little lower - trying to take existing protocols and build software that handles them all together in a better way. I also think it's a better approach than Wave, focusing on designing a single artifact that people will want to use instead of a framework technology demo, which is what Wave always seemed to me.
It looks like it's not intended to be a native desktop (or mobile) client - they mention running it on servers, and it depends on Python and CouchDB, which would preclude it running on the iPhone OS.
One of the things I like about it is the way they're trying to use a classification of emails - obviously, not all mails are equal. They fall somewhere in a range between spam and important personal (or work) messages. A big class is "bulk" mail - mail that's useful but not necessarily of a conversation involving you. Mailing lists, promotional mails you've asked for, facebook/twitter notifications might all fall into this category, and it's a great idea for a mailer to do something different with them. At the least, it should let you ignore them if you're just trying to do a quick processing pass of new mail.
The Raindrop design blog has some good posts (with plenty of screenshots) outlining their current approach.