Who would have thought that the humble browser URL bar would spark so much interest in the technosphere. When Ars Technica reported that prerelease version of Chrome sports a navigation bar that disappears when not in use, Mozilla Labs quickly responded with an experimental extension that offers a similar feature. ConceivablyTech followed up with an impressively long analysis of the topic, combining worthwhile insights ("once you are at your destination, there is no need to permanently display the address") with over-the-top rhetoric ("removing the URL bar will be most substantial change to web browsers since the release of Mosaic, the first publicly available browser").
There's no doubt that Chrome has pioneered a laudable initiative to strip the cruft out of the browser user interface, and Mozilla is eagerly following in their footsteps. Smartphones demand a complete rethinking of the browser UI, but tablets, netbooks and even larger laptops have limited screen real estate while still using a standard web browser. Even on a desktop computer, there's no point in using up space and distracting the user needlessly. Nonetheless, there are definite trade-offs inherent in an auto-hide URL bar: namely, it's harder to see what page you're on and to navigate to other pages.
In point of fact, it isn't the disappearing URL bar that heralds a revolution in browser user interfaces. It is, rather, the recognition that the one-size-fits-all web browser is fated to eventual extinction. In some cases, like when you are researching a topic by actively surfing from site to site, the URL bar is a key part of your browsing experience. In others, like when you are reading your webmail, it's totally useless. WebRunner hides the URL bar by default for exactly this reason: it addresses a use case where it distracts and takes up space without providing any compensatory benefits.
We can hack off as many bits of the browser UI as we want, but we're never going to hit on an optimum solution until we accept that there is no optimum solution. When browser vendors start to offer different browser user interfaces for different tasks, then we will be able to make intelligent decisions about which of the tradition UI widgets are needed and which can be left on the cutting room floor.
Update: Wolfgang Gruener points out in the comments that the Ars Technica article I cited was following up on a post on ConceivablyTech.