As of 12/31/2009 the test certificates that shipped with Windows Mobile SDK expired. Microsoft have generated new certificates that you can drop in to rectify this problem. If you’re a developer you can put these certificates right into Visual Studio (or sign via command line). If you’re a tester, you can follow these instructions to create a provisioning file for a device, so that you don’t need Visual Studio to push certificates onto test platforms.
Download: New Certificates