In sigs, especially HERE, when those posting are obviously webmasters who I assume want their legit links known -- and ranked higher in search engines, while at the same time promoting UBB (who's denying that "Google Juice" (haha, love that term)) -- then nofollow needs to go...
But in general, nofollow needs to go for user-links because even if it was that 1/10000 (or whatever lol) chance that it is real spam, admins/mods could delete the post before Google (et al.) even notice.
But I DO agree with nofollow for links pointing to any pages that require user input (Reply/Quote/etc).
