I am all for no follow signature links.
Go for it!
You could nofollow all signature links to discourage the spammers. That would not affect members looking for traffic. It appears to me that most spammers are either link sellers or idiots who just read a book titled Best SEO Practices for 2005.
If you send a welcome email to new members, you could mention that all signature links are nofollow, which neuters them for SEO purposes. I think DP went to nofollow links a couple years ago, except for paid members.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin
DP did go no-follow, but oddly it didnt seem to reduce the sig spammers... at least last time I was there. People following outdated SEO tactics arent known for paying attention to detail I guess. I might give it a shot... doesnt matter to me one way or the other. Dont see a lot of advantage to the links aside from occasional direct traffic and it cant affect that.
-- Do not meddle in the affairs of dragons, for you are crunchy and good with ketchup. --
I don't use signature links for SEO, just for direct traffic. No-follow wouldn't bother me if you decide to use it.
As long as there are functioning admins and moderators inside the respected forum site and with the help of the concern and active members, spammers will always be detected and penalize.