Starting yesterday, Microsoft has starting implementing a minimum requirement of Windows Phone 7.5 operating system to download apps from Windows Marketplace.
In a blog post last month, Microsoft said that requiring Windows Phone 7.5 is part of a larger effort aimed at improving Marketplace performance and security, and paving the way for even faster growth and more new features.
If you don’t update to Windows Phone 7.5, you’ll still be able to browse Marketplace. But you’ll get an error message if you try to install an app or game, or update one already on your phone.Visit this page if you are not sure on how to update your Windows Phone 7.
Windows Marketplace now has over 80,000 apps and is available in over 60 countries around the world and in Malaysia.[Source]– Windows Phone Blog