Vesper, Q Branch's note-taking app for iPhone that I reviewed last year, added sync support with version 2.0, available today on the App Store. Q Branch is calling the (optional) service “Vesper Sync”, but right now it's primarily intended to serve as a backup for your notes because Vesper is still iPhone-only (of course, there are also people who use two iPhones).
I've been following the development of Vesper Sync through the excellent Vesper Sync Diary series on Brent Simmons' blog. It sounded like Q Branch wanted to ship fast, reliable, and invisible sync with the app and that's exactly what they did. I like how Vesper Sync updates in (almost) real-time and how it doesn't feel like something that you have to manage inside the app – you only see a network spinner for a second in the status bar and then sync disappears in the background. It's a pretty sweet and elegant implementation, and it bodes well for Vesper's likely future availability on more platforms.