As indicated by Peter Foot’s post on Expired Development Certificates the certificates that ship with the Windows Mobile 6 SDK have now expired. So you either need to play around with your system clock or, you can now download (yes I know, yet another Windows Mobile development download) the new certificates from the following blog post http://windowsteamblog.com/blogs/wmdev/archive/2010/01/12/new-windows-mobile-developer-certificates.aspx