Microsoft has more or less pulled the plug for Windows Mobile 6.x. After May 9, 2012, no one can buy or download apps from the Windows Marketplace for Mobile 6.x app.
According to a post at Microsoft Answers
March 8, 2012
Dear Windows Mobile 6.x customer:
Microsoft is discontinuing the Windows Marketplace for Mobile service for Windows Mobile 6.x. Please review the details below to familiarize yourself with the changes:
Windows Mobile 6.x Marketplace Service To Be Discontinued
Beginning May 9, 2012, the Windows Mobile 6.x Marketplace service will no longer be available. Starting on this date, you will no longer be able to browse, buy or download applications directly on your Windows Mobile 6.x phone using the Windows Mobile 6.x Marketplace application and service.
Applications and games acquired from the Windows Mobile 6.x Marketplace service installed on your Windows Mobile 6.x phone will continue to work after the service is discontinued on May 9, 2012. However, additional downloads of these applications and games from the Windows Mobile 6.x Marketplace service will no longer be available.
Microsoft recommends you review the applications or games installed on your Windows Mobile 6.x phone and install any available updates in advance of the Windows Mobile 6.x Marketplace service shut-down on May 9, 2012.
Please note: All phone data, including applications acquired through the Windows Mobile 6.x Marketplace service, data and user-specific settings, will be deleted from your Windows Mobile 6.x phone in the event that your phone is wiped or "hard reset." Any data on a removable storage card, if present, will also be deleted if your phone is wiped or "hard reset".
Windows Mobile applications and games that are compatible with Windows Mobile 6.x may still be available directly from their developers or via third-party marketplaces.
Thank you for your continued support of Windows Phone.
-The Windows Phone Team
This moves comes as step to push people from legacy systems to Windows Phone 7.
Microsoft launched the Windows Mobile 6.x Marketplace in 2009 as an answer to Apple’s App Store, which at the time had some 35,000 applications (and now claims more than 550,000 apps).
The closure of Marketplace for Windows Phone 6.x doesn’t mean you can’t enjoy your legacy phone any longer. You can still download apps from third party app stores or directly from developers’ websites, just not from Microsoft’s store.