The newly opened store has 800 apps, compared to 12,000 on the Android Market and 100,000 on the iPhone App Store Microsoft is making its Windows Marketplace for Mobile available to phones running older versions of its mobile software, although not all of the apps may be available to all Windows Mobile users — showing the highly fractured state of Windows Mobile.On Monday, Microsoft said users of phones running Windows Mobile 6.0 and 6.1 can now shop for and download apps from its Marketplace. The Marketplace was initially only accessible by users of Microsoft’s most recent software, Windows Mobile 6.5.It also said that the store now has 800 apps, triple the number available at the launch of the store in October. But not all of those are available to everyone. Microsoft’s Web site that lets anyone browse through the Marketplace has just 376 applications. “People may not see all of them on the Marketplace Web site or smartphone catalogue, either because of regional access or because certain apps have specific device requirements such as GPS, screen sizes, etc.,” said Todd Brix, senior director of mobile services and platform product management for Microsoft.The discrepancy between the total number of apps and the number of apps in the online store demonstrates the downside to a business model like Microsoft’s, with a manufacturer-customizable OS that can work differently on different phones. That business model allows users the luxury of choosing the phone design they prefer, but it comes with limitations in interoperability.Google’s Android operating system also runs on phones with different form factors, and device makers can customize the OS. The Android Market has 12,000 apps and so far doesn’t seem to have significant issues with application interoperability. One reason is that the biggest customization of Android — as seen in the HTC Droid Eris — has been accomplished through the use of widgets that run on the standard Android OS, leaving the actual OS alone. Apple is on the other end of the spectrum, because it makes both the software and the hardware and also runs its app store. That vertical integration is at least part of the reason that there are now 100,000 applications in the iPhone App Store.Microsoft says there are more than 18,000 commercial applications available for Windows Mobile. Developers of those apps must submit them in order for them to appear in the new Marketplace. Otherwise, they are only available through third-party sites.Nancy Gohring is a reporter for the IDG News Service, an InfoWorld affiliate. Technology Industry