Doesn't seem to deter a large proportion of spammers, either--- I guess spamming is cheap enough that you might as well shotgun it, instead of bothering to research each site's nofollow policies. If you ever look at Slashdot's submission queue, there's a ton of spam in there too, despite the fact that it gets pretty quickly buried, is nofollowed on a robots.txt-prohibited page that not even many humans visit, and has no chance of being posted by the manual story-posting process.
I also think HN regulars are not the kind of the people that spammers could use... still it'd be pretty annoying during the period they learn that (the hard way)
One assumes that a core characteristic of spammers and spambots is that they ignore Robots.txt - or at least I would be extremely surprised if disallowing them in robots.txt has any effect.
