I also have the same issue with certificates and MfE synching.
It seems clear to import the self certificated cert. from IE7 like described in http://fwd4.me/Yy7.
The import seems to work well, the N900 accepts the certificate and I can see it in the N900 settings (Cert manager on the top of the list under the group "user" above "certification sites").

Unfortunately MfE syncing still doesn't work. It remains with the same error message something like "exch server needs a secure connection or the account is deactivated".

It seems, that the certificate is there, but the Mail Client doesn't see it. A problem of rights maybe ?
Read something about CA-field but couldn't figure out that with the German Browser.

I checked the hole internet and I'm lost now. The iphone works well

Have Swiss/German language with PR1.2, MfE 2007

Add Note: when using the mail wizard MfE in the N900 it comes with an error that says the certificate is not valid (as if it would not be there) with options to stop or ignore. I assume it should say nothing as the cert. is already imported.
Ignoring will lead to the above mentioned message

thanks