Anyone with a Windows Phone that hasn't been upgraded to version 7.5 won't be able to use Microsoft's Marketplace app store as of today.
Microsoft announced today that in order to download, buy, update, or review apps in Marketplace, users have to be running Windows Phone 7.5. This change is required for both the phone and web Marketplace storefronts.
"Most phones are already running Windows Phone 7.5, which was released last fall, and so most of you won't notice anything different about how the Marketplace works," Microsoft's director of program management, Mazhar Mohammed, said in a blog post today. "However, if your phone has an earlier version of our software installed, you'll soon start seeing an error message when you try to download a new app, or update one you already own."
Over the past year, the software giant has been pulling out several tricks to try to keep up with Android's Google Play and iPhone's App Store. Earlier this month it announced an app developer summit to happen in San Francisco this June and last summer it began paying app authors for their Windows Phone attention. As a result, Marketplace's available apps spiked from 30,000 in May 2011, to 50,000 in December of that year, to 80,000 this April.