But also there is an indirect human utility to having botnets being used for mining - it is a very benign payload compared to other things criminal hacking activities have historically used botnets for.  Maybe spam would even fall if hashcash CPU/GPU mining is a more profitable market than spamming.  It seems to me highly likely that it would be even.  Maybe hashcash beats spammers yet in an incredibly indirect and unexpected way   Thats amusing.

Another defensive thought: bitcoin may like to take a leaf from mondex, p2p respendable electronic currency cash card.  They had a hot spare crypto protocol ready and predeployed switched on via peer2peer transfer of signed upgrade notice cards in case of cryptographic or implementation problem.  In a bitcoin world that might more be a spare implementation in another language or something.  (Its a common concept in mission critical systems eg spacecraft navigation computer, to have two or three different implementations in different languages, by different programmers, but from the same spec, voting on what is the correct reaction and course adjustment).