Looks like Google has finally added a much needed feature to the browser, an alert while closing multiple tabs. This feature was hugely in demand but was ignored by Chrome for a long time. I had written on ways to restore accidental windows closures in Chrome and also about an extension which alerted you when closing a tab.
However, the latest dev version of Chrome 9.0.597.15 dev, seems to have this feature built-in. The request for adding a popup notification was submitted back in September 2008, however, it was ignored for the past couple of years.
Chrome also provides an option to turn off multiple tab closure warning through a checkbox option. However, I did not find any options to turn this feature back on. I will keep looking and update this post when I find it.