The full math is here - David you would probably be interested in this if you haven't already seen it.
http://www.bitcoinunlimited.info/resources/1txn.pdfThe paper also describes how the sigops attack is mitigated through miners simply mining 1tx blocks whilst validating then pushing that out to other miners whilst they are still validating the 'poison' block. Rational miners will validate the smaller block, and they also be able to mine another block on top of this, orphaning the poison block.
The attacker would get one shot, and would quickly be shut out. If you have enough hash rate to be mining blocks yourself its really much more profitable to behave!
Thank you so much sgbett; that was a really great read. I must admit I didn't work through all of the math yet but at first blush it appears ok until;
The Bitcoin network is naturally limited by block validation and construction times. This puts an upper limit on the network bandwidth of 60KB/sec to transmit the block data to one other peer.
Hmm, really? There's no way ever to improve on block validation times? Quantum computers? Nothing? That doesn't ring true.