This is a helpful link to explain how to remove a security certificate by DigiNotar CA. I can’t get the full story on what’s going on, but apparently this is a spoofed certificate that affected people using Google services. Near as I can tell this was some attempt from someone in Iran to spy on dissidents using Google to organize protests.
The certificate has already been revoked, and an upcoming browser release will revoke and delete the certificate. But if you’re in Iran (and I actually do have a fraction of traffic from there*) it might be a good idea to delete this pronto for your own safety. So here is a Firefox article explaining how to do it. The rest of you, I doubt the certificate will do anything bad since it’s already been revoked, but if you want to remove it, the instructions are fairly straight forward.