Posted by: Brett
Beginning May 22nd, which was this Tuesday, the Windows Phone Marketplace has started requiring Windows Phone 7.5 to access and download apps from the Marketplace. Most of the Windows Phone devices support v7.5 so it shouldn’t be an issue for most. Microsoft says the requirement is to enhance security and speed so I guess it’d be a good idea to update if you would like to continue downloading apps.
Source: Windows Phone Blog