You cannot build a botnet large enough to out-hash the army of dedicated ASICs that's responsible for todays' Bitcoin hash rate. Not even a botnet that merges all personal computers in the world would be able to do that - ASICs are just that much faster.

But - some miners and mining pool operators are rumored to be among the bigger Bitcoin whales themselves. They already own (or at least control) a large part of the ASIC hashing power. None of them is even close to 51% today, but then you also don't really need 51% to pull off a double-spend attack, 30-40% should do it, as long as you pull that hashing power from the legit network at the same time. A cartel made up of the top miners and mining pool owners should be able to comfortably reach this critical mass of hash power. So effectively it comes down to whether they assume that it's better for them financially to continue supporting the networks' health or to pull a one-time heist, destroying the networks' credibility in the process. For the moment, the former seems to be still true. But nobody knows for sure whether that may change some day...