Click on the Apple when the focus is on Finder, and Right under Software Update is "Mac OS X Software", which directs to http://www.apple.com/downloads/- essentially the App Store for desktops. It doesn't go through the wretched iTunes interface, though, and I don't believe applications are for sale, only free download.
I'm sure a paid download and sales model is in the works, or of course they've at least thought of it.