If we assume the average computer in the BredoLab botnet (largest ever known) has a time-averaged hashing capacity of 10mhash/sec (very rough number considering the high end hardware/gaming market is a few percent, likely has an inverse correlation with botnet membership, and most zombies run at less than 100% uptime), then a botnet as a whole of that size could do around 300thash/sec, increasing hashing power by 5,000% and netting the botnet owner somewhere in the neighborhood of $11 million a day. (wrongly assuming this has NO effect on the exchange or difficulty rate)

This of course would give said botmaster a stranglehold on the bitcoin economy, he could monopolize generation by producing 98% of all blocks, he could hoard his coins to drive up prices.

The one thing he couldn't do is sell a large lot of them without crashing the market. Realistically moving anything upwards of a quarter million a day is going to have a large deflationary impact on the market in short order.

Feel free to check my math, but even a fairly small botnet could wreak havoc.