Given the short timeframe, Microsoft has chosen the most direct approach to developers' hearts – cash. By providing them with free test devices and tools, covering development and marketing costs, or straight out guaranteeing app revenues, Microsoft outlays more financial incentives than it ever did with Windows Mobile. At least four of the big names in applications development have been approached and given the terms of the incentives, despite that they have their hands full with iOS and Android.
Windows Mobile had the best selection of third party apps before the iPhone altered the playing field, but they are mostly outdated, and not compatible with WP7. We can be fairly sure that right out of the box, Windows Phone 7 devices will join the immensely popular Microsoft ecosystem (Microsoft Office, Windows Live, Exchange, Hotmail, Bing Maps, Zune, Xbox Live), and play nice with your Windows rig. To reach the state of “there's an app for that”, however, Windows Phone 7 will have to attract developers, and Microsoft seems to be fully aware of the stakes.