Microsoft have just announced that starting today, you'll be required to have Windows Phone 7.5 on phones to download, buy, update, or review apps in Marketplace. This change applies to both the phone and web Marketplace storefronts.
While this seems like a strange move, remember that most phones are already running Windows Phone 7.5, and most phones can be upgraded to the latest version if they're stuck on a previous version of the operating system.
If you're running an older version of Windows Phone and try to access the Marketplace, you'll be prompted with an error message when you try to download a new app, or update an app that's already on your device. If you do get the error, all you need to do is install the free Windows Phone 7.5 upgrade.
You can find more information, and a step-by-step instruction guide on Update Central.