If you've been using the sample certificate from the Windows Mobile 6 SDK then you know that it expired on the 31 December 2009. There's lots of questions around this about when Microsoft will fix it and what to do in the mean time.
When will Microsoft Fix This
A fix from Microsoft is coming in the next Windows Mobile SDK. But a good thing is that a fix isn't really needed. The certificate that comes from Microsoft is more of a convience than a necessity.
What can I do until Microsoft provides a fix
You can make your own certificate and provision your device with it. Mal Loth pointed out in this MSDN Forums Thread that any one can create their own certificate and install it on their device. He refers to this CodeProject.com article that contains the information on how to do this.
CodeProject
Tags: Windows Mobile