Today’s the day. This afternoon we started requiring Windows Phone 7.5 on phones to download, buy, update, or review apps in Marketplace. These changes will make Windows Phone OS 7.5 the minimum OS required to install new or update existing apps from Marketplace via the phone or Web.
If you are wondering why, I invite you to read this article - http://windowsteamblog.com/windows_phone/b/windowsphone/archive/2012/04/24/two-marketplace-changes-and-how-they-affect-you.aspx
If you do get the error, all you need to do is install the free Windows Phone 7.5 update. You’ll find all the information you need to do that, including an update checklist and easy step-by-step instructions, on Update Central. (Not sure if you have Windows Phone 7.5 installed? Here’s how to check.)
By the way, even if you don’t use Marketplace, there are still lots of great reasons to update. We made hundreds of improvements, big and small, in Windows Phone 7.5
Source : Windowsteamblog