A default installation of Mono doesn't trust anyone!
mozroots will download and import trusted root certificates from Mozilla's LXR.
Read the Mono Security FAQ
EDIT:
Try the latest version of Mono packages from here
If that doesn't work either try the SVN trunk version here
From the Bug 606002, Gonzalo Paniagua Javier' suggested: "The way to go is to add your ServerCertificateValidationCallback to ServicePointManager and ignore errors with this code."
I have little experience in this area so I suggest to contact him for further assistance, maybe he can help. He's email is gonzalo@gonzalo.name (his blog)