As previously announced by Microsoft, Windows Phone 7.5 is now required on your Windows Phone device if you want to interact with the marketplace in any way. This interaction includes downloading, buying, updating, and even reviewing Windows Phone applications on the marketplace. The Windows Phone 7.5 update is required for interactions both on the web marketplace and a device itself.
These changes will actually take a day or two to take full effect, according to Microsoft. However, if you in fact have a device that is not updated to the required OS version, you will receive an error message when you try to interact with the Windows Phone marketplace. Regardless, Microsoft urges all users to make sure they have the latest OS version due to the many features that have already been added, and improvements made.