If you're having trouble downloading apps from the Windows Phone Marketplace, you just might be a pre-Mango Windows Phone 7 user. Microsoft is now enforcing a rule that only users of Windows Phone 7.5, codenamed Mango, can use the Marketplace in any meaningful way. Without the Mango update you can still browse the Marketplace, but you will start getting error messages if you attempt to download, buy, update, or review an app.
Microsoft originally announced the switch in April. At the time, Microsoft said it needed to enforce the Mango update to improve Marketplace stability and security, as well as making way for new features and improvements to the Marketplace.
It may take a few days for the new change to work its way through the system. So if your non-Mango phone hasn't been shut out of the Marketplace (except for browsing) yet, it soon will be. The majority of Windows Phone users have already upgraded to Mango, according to Microsoft, so the change should only affect those reluctant to make the Mango switch.
Microsoft originally launched Windows Phone 7.5 in September for all Windows Phone users offering new features such as improved voice commands, expanded HTML 5 support, built-in Facebook messaging, improved live tiles, and SkyDrive document editing.
Microsoft's attempt to force stragglers to update to Mango comes as Windows Phone fans wait for the company to introduce Windows Phone 8, Apollo. The next significant version of Windows Phone is expected to offer deep integration with Windows 8, the upcoming desktop OS due out in late 2012. There is also some concern that Apollo may not be compatible with current Windows Phone hardware; however, that is only speculation and rumor at this point. Before Microsoft releases Apollo, the company is expected to release another minor update to Windows Phone 7, codenamed Tango.
If you haven't upgraded your phone to 7.5 yet, you can find detailed instructions on Microsoft's website.