UPDATE: it seems that most other phone brands and types work as well (the S40 based devices are left out... sorry).
All you need to do is make sure that the certificate is in the DER format. The webpage doesn't verify if the certificate is in the correct format. This is up to the uploader.
I created this page, because I work a lot with digital certificates, so I don't want to be bothered with the workaround described in the earlier post. The current version is quick-and-dirty (no error messages). I'll try to make it more user friendly in the next couple of days (like having the option of sending the URL to an e-mail address). Just make sure that you obey the guidelines shown on the page, and all should go well.
Feel free to add a comment on how to improve this.
UPDATE: This works on (almost) every Symbian based (Nokia) phone. It has been tested with a couple of phones from the Nokia E and N series.
UPDATE 2: There is something else you need to consider when trying to add a certificate to your phone. Ideally, an SSL certificate is issued by a Certificate Authority. You can verify this by comparing the 'issued by' and issued to' filed in the certificate. If those are the same you've got a selfsigned SSL (or CA certificate). Those should work when you upload them. If these are different you need to get the CA that issued the SSL certificate.
If the issued by and issued to field are different it means that this is NOT a ROOT CA certificate or self-signed certificate.