In Microsoft's own words...
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.
So what does this mean?
If your WindowsPhone is running a software version that is less than 7.10.7720 (Windows Phone 7.5), you will no longer be able to access the Windows Phone Marketplace. The solution is simply to install the free Windows Phone 7.5 update. A guide to updating to Windows Phone 7.5 ('aka 'Mango') can be found at the Microsoft site.
This is a good demonstration of the need for Microsoft to look at incorporating OTA updates into Windows Phone, as to do the above upgrade you will need a Windows PC, or a Mac. I'd even like to see the option to upgrade via plugging your device into your Xbox, negating the need for a computer.
[Via: Windows Team Blog]
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