Google chrome developer builds for Mac OS are available right now but still the beta version is not out, and the first stable beta version of Google chrome browser is coming out in December.
For windows users, its 4.0 beta available but for Mac and Linux users, they are still limited to use the developer preview. The new mailing post suggests that the beta for chrome users is going to come in December, so not far before Mac users download and check the user-interface.
Windows users have been seeing the development like the latest addition of extension feature, and also another technology called Browser Action where people can interact with the extensions, through a small button in the upper right corner of the browser.
Google chrome team is trying to include the Extensions and Browser Action feature in the first beta release itself next month. You can download the developer preview till then to acclimatize yourself with the browser in Mac OS.