As a CAcert user I need to install CAcert's root and class3 sertificates to all devices I use. I tried to install sertificates to N900 like I had used to do with S60 phones, as it appears N900's browser goes mad when I pressed hyperlink to .cert or .der file.

From the above I realized that filemanager might be the thing as Cert manager just happily shows what you got and browser does nothing clever. It looks like filemanager supports both PEM and DER formats of certs and installs them without much of a strugle.

I think that settnigs Certificate manager is misleading, as it really does nothing of management, but it's very good that certificates can be installed through file manager