interesting that they were able to identify that there was bug without being able to say what it was or how it happened.
But without knowing the bug, there is still the enormous vulnerability; pretty much anyone else could do what they have done, crack these keys, and MITM any of the sites that use them.
So, they could have kept quiet, tried to track down who had generated the keys, try to track down how they had generated the keys, and see if there is a problem. But the longer they do that, the more people they'll have to contact, the more likely it is that word will leak out, and the more likely it is that someone nefarious could use this information to their advantage.
By making a big announcement about it, they let everyone know now. Everyone knows to start looking at these keys. They will likely notify any CAs that they can identify of the bad keys that they have found so that they can be revoked (though they have mentioned that finding contact information has sometimes been difficult). Yes, it means that we don't really know how this happened; but it does mean that a lot more people will probably start auditing their entropy sources and pseudo-random number generators to make sure they are working properly.
But as far as I can tell, the result is that, of public keys collected "in the wild" show more duplication than should be expected, so the random number generation schemes used are not "random" enough. Is that it, or is there more to the story?
(Edit) No, there's more. I don't quite understand (and don't think they really explain), but somehow they use the fact that there's lots of keys to find ones with common factors, which allows you to factor both of them. This sounds basically like the Euclidean algorithm but I couldn't tell what method they're using to find them.
So what the researchers did, apparently, was to gather all the RSA public keys they could find (6 million or so) and then calculate the gcds of all pairs of keys. (Presumably they were slightly more clever than this, but it doesn't really matter--doing this is still vastly easier than factoring even a single public key). Whenever they found a gcd that wasn't equal to 1, they'd cracked (at least) 2 keys. And it turns out that roughly 0.2% of the keys could be cracked in this way (for a total of 12720 1024-bit moduli).
As far as I can tell, the problem isn't with RSA itself, but with whatever algorithms are used to pick two random primes to multiply and produce the key (or moduli, whatever)--they don't have high enough entropy and output duplicates ~0.2% of the time.