I've considered doing nofollow on PM -- specificly, for links made by anonymonk, and users with only a few XP. (Links made by users would loose their nofollow if the user gained XP.)
Just because it hasn't been implemented yet doesn't mean that nobody is considering it. While comment spam is low on PM, it is decidedly nonzero. As for the naysayers here, I've considered your argument, and think it's bunk. 1) I know Evan, who invented it, and trust him. 2) Even if it doesn't help us much, it doesn't hurt us, and does hurt comment spammers. 3) Email spam and comment spam are very different things, and shouldn't be considered the same. 4) It doesn't hurt legitimate users of the site; legitimate users get accounts and gain level 2 fairly quickly... and even if all external links were marked nofollow, I still don't think that really hurts anyone, with the possible exception of the people being linked to.
Warning: Unless otherwise stated, code is untested. Do not use without understanding. Code is posted in the hopes it is useful, but without warranty. All copyrights are relinquished into the public domain unless otherwise stated. I am not an angel. I am capable of error, and err on a fairly regular basis. If I made a mistake, please let me know (such as by replying to this node).
