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According to Engadget, the Marketplace opened one day ahead of schedule; it allows users to download one of 20,000 Windows Mobile applications directly from their phones. It's nice to see Microsoft hurrying things up a little, especially when you know that over 30 smartphones running on Microsoft Windows Mobile 6.5 should be launched before the end of the year.
In typical Microsoft fashion, the place where you find out more about Windows Marketplace is this link - developer.windowsphone.com/Marketplace.aspx - while www.windowsmarketplace.com refers to the discontinued software delivery platform Windows Marketplace (as opposed to Windows Marketplace for Mobile), and is currently just a reference site linking to Microsoft Store.