Microsoft is set to reach another mobile milestone Tuesday as its Windows Phone 7 Marketplace passes the 5,000-app mark. The Marketplace reached 4,000 apps just 12 days ago, and it took 20 days to jump from 3,000 to 4,000. At its current pace, the Windows Phone 7 Marketplace could overtake Palm’s webOS App Catalog as soon as next month. Microsoft announced last week that Windows Phone 7 devices sales have topped 1.5 million units globally, and developers clearly see the potential in the company’s emerging platform. Currently, more than 18,000 developers have registered to build apps for Microsoft’s new mobile operating system.