One of the pains many administrators have been having is the use of test certificates and the use of non-standard issued certificates. Many times the bargin SSL Certificates (FreeSSL, GoDaddy, etc) work fine on Windows XP but fail to function properly on the mobile devices. This wasn't much of a problem with Pocket PC 2003 because you could just run DisableCertChk and ignore the problem. This work around has been removed in Windows Mobile 5, however some enterprising people have found a way to add the a certificate even to a locked WM5 device. For a run down of your options see this thread:
http://geekswithblogs.net/wallabyfan/archive/2006/04/19/75608.aspx
For a list of OS to Certificate support click here:
https://partner.microsoft.com/global/partner/40027352
- Joel
For a Microsoft authorized way to add root certificates:
http://support.microsoft.com/?kbid=915840&SD=tech
Microsoft also recommends adding the certificate using a CAB file. This method has worked for me on smartphones that won't let you modify the registry
https://blogs.msdn.com/windowsmobile/archive/2006/01/28/making_a_root_cert_cab_file.aspx