At the moment you would need about 30GH/s to get approx. 1 block a day or 50 btc a day.
With how difficult it is to install opencl with an unattended installation, most botnets would be using CPU mining on their bots, probably getting 1 MH/s a pop
to get 30GH/s, or 30,000MH/s, a botnet owner would need (rather obvious now) 30,000 bots running a miner to start getting a steady income (with a mining server all their own). Not only that, but 30,000 connections is a bit much for a standard server, so the owner would require a massive server just to manage all the shares.
Until someone figures out a way to harness GPU power without complex opencl installations, it just isn't feasible. (unless of course the owner used an already mining pool that doesnt require registration, and then send all the coins to himself, but thats just ABSURD
)