All sites needs incoming links to grow. But a lot of places where you can get those links have a nasty habit of having no-follow tag.
In twitter for example, when you specify your web address (that’s showing you in your profile page) – http://www.twitter.com/cferseta, you can see a “no follow”.
In blogs comments – no-follow (actually, this is good, because anyway a lot of comments are spam ones)
Delicious – no follow
Stumbleupon – use redirects and frames
DIGG – follow! 🙂
Google say it’s not taking into consideration no-follow links. But did he? My impression is that up to a point it takes them into consideration, even it backward links command does not show it.