Two important Windows Phone Marketplace changes that affect end users (via the Windows Phone Blog)
In short, first, we’re removing the option to shop for Windows Phone apps from the Zune PC software. Second, as part of a larger Marketplace improvement effort, you’ll soon need Windows Phone 7.5 installed on your phone to buy and download new apps, or update existing ones.
Make sure you have updated your Windows Phone device to Windows Phone 7.5 by connecting it to the Zune software and selecting “Settings” à “Phone” à “Update”. Before you start the process, which can take an hour or more, you’ll want to read over the checklist on Update central, our comprehensive how-to and troubleshooting resource. To install the update, follow the steps in this article.