In a blog post today, Mazhar Mohammed of the Windows Phone team announced that Windows Phone 7.5 (Mango) devices are now required to access the marketplace.
In the post, Mohammed confirmed that in order to download, buy, update or review apps in the marketplace, users must now use a handset featuring Windows Phone 7.5 or newer. The new marketplace requirement applies to both the marketplace accessed via the phone and the online, site-based marketplace. Mohammed had previously announced the requirement in April, but the requirement did not go into action until today.
The change may not go into effect for some phones until the next few days; after that period, users who do not meet the requirement of having a Windows Phone 7.5 handset will be subject to error messages when attempting to download an app or update a previous app. The error message will go away as soon as the user installs Windows Phone 7.5 or newer.
According to Mohammed's earlier blog entry, the marketplace is beginning to require Mango devices due to performance and security concerns. As the marketplace continues to grow, Microsoft is attempting to "[pave] the way for even faster growth and more new features," Mohammed wrote. Users who don't upgrade will still be able to browse the marketplace, but no interactive actions will be available to them.
Microsoft's step-by-step guide to updating a Windows Phone handset can be read here.