Would any of the current botnets be able to launch and sustain a 51% attack?
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.