Would any of the current botnets be able to launch and sustain a 51% attack? 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?
If you want to profit from Bitcoins, you would not want to work against it (as was explained by Satoshi in his original paper). A 51% attack would not bring you profit, rather it would make everyone involved in the project loose, as Bitcoin's reputation would be damaged.

The other thing is getting enough hashing power. Unless you target mining-specific machines, chances of getting higher-end mining-compatible graphics cards isn't too high. So the trojan would have to rely on getting a lot of moderately powered PCs, and in order to launch a 51% attack, the botnet would have to be quite big. The biggest botnets, consisting of over 100'000 computers could potentially accomplish this, but so far Bitcoin isn't a prime target for botnet activity.