If you want more insight ask Tycho what kind of load even small botnets do to a pool server. Now with enough computing power (we are talking a dozen or so high end load balanced servers on a 1 Gbps connection) a 1.8 million node botnet "could" operate in a pool-miner model. However now compare all that cost vs the incremental reward of .... ready for it ....about 2 cents per block.
This is an excellent argument for pools disallowing clients which aren't turning in enough results to justify their bandwidth. If low hashrate clients (botnet or not) want to participate, they can have one guy get work and distribute it to the others. Letting them collect 50BTC per null block to save the pool the awkward responsibility of turning away unprofitable workers is completely backward.