First off, the spamming is most obviously automated. At this point, it's still going to go on anyway. Second, at least on the .Text system, we can moderate or delete the comments anyway (which we know about instantly with the e-mail notification). The stuff doesn't stick around long enough to ever get indexed.
People are acting like there has never been spamming on the Web in cases where anonymous posting can take place. Give me a break. It has been going on for as long as discussion boards have been around. The only reason anyone cares now is because everyone has a blog that they own, unlike the countless message boards they visit but don't own. I especially find it funny that some people go as far as to say, "My blog is important and visited by millions, and a single link can skew page rank!" Wow.
The whole discussion misses the point: Free advertising is free advertising in the mind of spammers. It doesn't cost them a dime to send out thousands of e-mail messages, and it won't cost them a dime to spam thousands of blogs. I don't have the answer, but this certainly isn't it.