They're not diving very deep to find apps for this one (MLB At Bat 2010 is the iPad app, and We Rule is the iPhone choice -- both of those hardly need any more promotion), but since we've heard from developers that Apple promotion really can make or break an app, the addition of another app spotlight can only mean good things for those apps.
Critical Thought's David Whatley made this point well at GDC a few weeks ago -- he said that niche outlets (like our own little blog right here) covered his app first, which then gained the attention of Apple, who then promoted it in the App Store as "new and notable" and on the top paid charts. That then led to mainstream attention, which of course led up to major sales numbers. In short, the more Apple promotes apps, the more they sell. And that's why an "App of the Week" promotion like this can mean a lot to the right developers. MLB doesn't really need the extra spotlight, but getting a slot in the new App of the Week spot could make the careers of quite a few smaller developers out there.