We've always heard the constant cry for a web-accessible version of the Marketplace, especially those who use OS X (like myself) -- the Windows Phone 7 Connector is simply no Zune media center. Matt Bencke, in the same post where he announced the Mango SDK being available, spoke about plans to bring in a web version of the Marketplace.
"To better reach customers worldwide, I’m also pleased to announce today that Mango will offer a new web version of Windows Phone Marketplace. This will enable customers to shop, share and buy/download apps and games from any PC and send them directly to their phones. You get more visibility for your apps with no extra work. The Mango Marketplace will bring several new features and capabilities that Todd Brix will be expanding upon here a little later."
Great news all around. The ability to view and purchsse apps on your PC (or Mac -- hopefully) will surely improve the usage of the store and bring a new wave of users to WP7 developers.
Source: Windows Phone Developer Blog