The developers of Google Chrome have moved a branch of Chrome 4 to the Beta channel, meaning that the browser is stable enough now for a larger testing audience.
Chrome 4.0.223.16 beta is available only for Windows at this stage, but includes bookmark sync. Extension support is however disabled at this time (though is still available in the dev channel releases).
You can download the latest beta version from Google.
Below is a video from Google demonstrating bookmark sync.