I think a lot of commenters are missing an important point of the article. The whole thing was the scientific process in a nutshell: he made an observation (most blog spam is ie6), made a couple hypotheses (most ie6 traffic is bots, blocking ie6 will significantly reduce spam), tested them out, and shared the results. He was even able to make a different, related conclusion (numbers for windows and ie6 browsing are inflated) based on his work.
Overall, I found the piece quite interesting. I think you will, too, if you read it not as a piece of advice but as a nicely-performed experiment.
I think this is probably an example of using a hammer to crack a very annoying nut. If his usual users are all linux types anyway I suppose it probably isn't hurting much.
This is a horrible idea. The point of systems like Akismet, etc is to avoid doing horrible solutions like this which penalise users. His apology to IE6 users obviously means nothing to them, because they can't read it!
Overall, I found the piece quite interesting. I think you will, too, if you read it not as a piece of advice but as a nicely-performed experiment.