As already reported, Microsoft had planned to withdraw Marketplace support for pre-Mango phones, i.e. those not updated to Windows Phone 7.5, and today's the day. See the quote below from the official Windows Phone blog, along with links to follow if this affects you, unlikely though this probably is!

From the Windows Phone blog:

Today’s the day. This afternoon we started requiring Windows Phone 7.5 on phones to download, buy, update, or review apps in Marketplace. As I’ve written previously, this change applies to both the phone and web Marketplace storefronts. (If you’re just tuning in, please read my earlier post for the full details on why we’re implementing this new requirement.)

Most phones are already running Windows Phone 7.5, which was released last fall, and so most of you won’t notice anything different about how the Marketplace works. However, if your phone has an earlier version of our software installed, you’ll soon start seeing an error message when you try to download a new app, or update one you already own. (I say “soon" because it will take a day or two for the changes we made today to take effect.)

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.)

Anyone passionate about Windows Phone (I mean, you're reading this, right?) will no doubt have upgraded to Windows Phone 7.5 on day one (for your particular device/variant), but it's still worth being aware of this change in case you have friends of family with a less loved machine.