This would make it a more complete replacement for the tab bar, which I would prefer to hide, as vertical screen space is at a premium compared to horizontal space with widescreen monitors. And I'm the type of person that likes to see as many lines of code on screen as possible.
For a good example of this type of functionality, take a look at Espresso 2's UI (http://macrabbit.com/espresso/). Or even Chocolat (http://chocolatapp.com/).
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