We’ve run into an issue with the digital certificates used to sign apps, and this is preventing some phones from installing some apps published during the last couple of days.
We’ve investigated and determined the issue only affects phones that upgraded to Windows Phone 7.5 from an earlier version of the operating system. It does not appear to impact phones sold with Windows Phone 7.5 preinstalled.
We estimate the issue also affects only a small percentage of the 100,000-plus apps in Marketplace. Among the more popular ones affected
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.