A bug in the Windows Phone 7 Marketplace has resulted in many owners not being able to update or download apps. Microsoft says it is working on a fix, but as a temporary measure it is stopping uploads of new apps or updates to the Marketplace.
A post on the official Windows Phone blog says that the error only affects some apps, and only impacts people who have upgraded to Windows Phone 7.5 (so more recent Windows phone buyers shouldn't be affected). Developers won't be so thrilled with the more wide-ranging nature of the temporary fix:
We know what’s causing the issue and are already planning to roll out a fix. Until that’s finished, we’re going to temporarily stop publishing new apps in Marketplace. If you get an error message while installing or updating an app, the best thing to do is wait for us to correct the problem, which we’re working to do as quickly as possible. I don’t recommend uninstalling any apps on your phone, since you likely won’t be able to download them again until the issue is resolved.
Microsoft hasn't given a time frame for a fix.
Can’t download an app? Here’s why [Windows Phone Blog]