I also have problems with this. I'm using mobile certificate for business VPN network connection. This mobile certificate uses sim toolkit for authentication. I have discussed about problem with Nokia Care customer support and local mobile operator (Sonera). Last message from Nokia was something like:
"N900 will not support sim application toolkit because it has totally different operating system than earlier Nokia phones (maemo5). Because of that your mobile operator should develop certificate which will be supported by N900"

I really don't know what to say? WTF?!?!