As some of you may have noticed the development certificates which are shipped with the Windows Mobile 6 SDK all expired on 31st December 2009. There have been no updates provided yet so the only official workaround is to fiddle with the date on your development PC and devices to set it before 31/12/2009. A ray of hope has been cast by moderator "Chunsheng Tang" on the MSDN forums:-
"FYI. The issue will be addressed in the next SDK update in the near furture. Besides the workarounds mentioned in this thread, you can also temporarily change the date of the development PC and the devices backward (make sure it's before 12/31/2009)."