I just installed a new StartCom SSL certificate on my webserver and got this error, but it was only temporary.
Wikipedia about OCSP:
As I understand, this protocol checks the SSL authorities for revocation status of the certificate.
It answers that the certificate is not known, but this is probably because it is new and its existence has not yet propagated to the OCSP online database. Just wait and try again later.
IE and Firefox also support OCSP, but I guess they don't nag about unknown certificates, just revoked ones.