Certificate authorities seem to be a real weakness in SSL
My main reaction to recent events is that they show how certificate authorities are one of the real practical weaknesses in SSL. All of the theoretical security in the world can be trivially thrown away by shoddy practices on the part of a single trusted certificate authority, and we've recently seen not one but two such CAs exposed.
The case of the improperly issued mozilla.com certificate is a clear-cut procedural failure, either failing to vet your resellers or failing to properly vet a signing request or, for that matter, both flaws at once, depending on your perspective. My view is that it is both at once, since Comodo appears to have been happy to take money from what could generously be called an extremely dodgy reseller and to delegate customer verification to resellers, which I am not convinced is at all consistent with a CA's responsibilities.
While the RapidSSL CA exploit is primarily an attack based on the weaknesses of MD5, note that it was only made possible by the sloppy security habits of RapidSSL, namely sequential serial numbers and fast, predictable signing speeds. The latter is especially significant as a CA security issue, because it means that the ability to sign certificates with RapidSSL's CA certificate is very 'close' to the public-facing website and is fully accessible by automated means. Even if an attacker who can compromise the website does not get the CA private key, they have probably gained the ability to get a valid certificate for any host.
(I am going to generously assume that RapidSSL's internal software won't auto-sign CA delegation certificates or wildcarded certificates, and that RapidSSL's private key is held in a hardened dedicated crypto processor, not in something that an attacker could compromise.)
It would be nice if we could believe that these were isolated incidents. But I can't; I think that they are the inevitable end product of the kind of economic incentives that CAs operate under. There is simply no money to be made from security and from vetting people; security costs money, and vetting your resellers and customers not only costs money, it also loses you money as you have to turn down more resellers and customers instead of taking their money. In the absence of actual costs to this behavior (as opposed to occasional bad press), we should expect that there are far more weak CAs out there that just haven't been exposed yet.