From the creators of Things, a wonderful to-do manager for the Mac and iPhone:
Believe it or not, we have been working on over-the-air sync since the beginning of this year. In the meantime, our goal changed from “something that works” (check out our competition if you want to know what we mean by that) to “a really sweet solution”. The tough challenge here is to develop a highly performance-oriented solution that works for everybody. In particular, we wanted it to work for Mac and iPhone users alike, without requiring a MobileMe account (or any WebDAV disk for that matter).
The whole issue is indeed complex and interesting enough to warrant its own blog post, but suffice it here to say that we are well under way to providing the above-mentioned “really sweet solution."
I currently use Remember the Milk, mainly becuase my iPhone and their web interface sync no matter where I am. If the Things guys get this working, I’d be all over it.