The "owners" of botnets probably runs them as businesses, meaning that they will always seek profit-optimization. Do you believe that it is a matter of time before they will seek to launch and sustain a 51% attack on Bitcoins?
If one were to achieve a 51% attack, one would probably not get any significant amount of Bitcoins exchanged into ordinary currencies, before the value of Bitcoins would plummet. Also, I believe that at the moment, there are too few merchants that accept Bitcoins, so one would not be able to order anything of significant value before merchants would react to the fraud. Do you believe that the reason why botnets have not attacked Bitcoin, is that they would not have enough to gain, or is it because they do not have enough hashing power?