PLEASE add this feature :) google turns up allegations that in previous version ctrl+z did the trick, but it doesnt work on Safari 3.1. There is also some mention of a plugin that does this, but it's MacOS specific. Undo Close Tab should reopen the last closed tab and retain back/forward history. Thanks!
Tabs are a feature of Safari, the browser, not WebKit, the engine. Enhancement requests and bugs against Safari should be filed at <https://bugreport.apple.com/>.