Of course, I'm not going to downplay the effects this could have for Cisco and in the long run for possibly tainted opensource projects.. The comments in here speak for themselves that people can't keep their hands off the source-code.
I've seen the 12.3 sourcecode before, under NDA, and several institutions outside of Cisco has legal access to it. Several universities, most of the larger security-firms such as ISS and whatnot have had access to it for years. So it's been combed through pretty well before. Sure there might be an odd exploit released from this source, but I don't count it as very probable, and certainly not as a threat to internet stability.