The Windows Phone app download problem has now been fixed by Microsoft. It will take a day or two for the fix to deploy though. As first suspected the issue was related to digital certificates.

End-users don’t need to do anything once the fix has finished rolling out.

Read more about the issue and the information about the fix over at the official Windows Phone blog:
http://windowsteamblog.com/windows_phone/b/windowsphone/archive/2012/08/14/can-t-download-an-app-here-s-why.aspx