in a new blog post today, Microsoft emphasized that Windows Phone 7.5 will now be needed to buy, download, or update apps from Windows Phone Marketplace, both the phone and on the web. Final work on this requirement will be made soon and this update to WP 7.5 will be necessary in order to access the Marketplace.
"For the past few weeks I’ve been mentioning that you’ll soon need Windows Phone 7.5 installed on your phone to buy, download, or update apps from Marketplace. (This applies to both the phone and web storefronts, by the way.), Microsoft explains. If you want the apps or games, you will need Windows phone 7.5 installed. Microsoft will offer a final notice before the requirement becomes mandatory.
(UPDATE) MAY 22nd - This requirement is now mandatory.