MHes - 12:45 pm on Dec 13, 2012 (gmt 0)
ken_b - I think pr from other sites is a lot less important than it was, so the risk of losing a few inbound links by putting nofollow on your outbound links, is not as important as the risk of linking to a bad neighbourhood.
JesterMagic - I would definitely err on the side of caution, in fact, reverse the logic and assume all bad unless they are very high quality sites. Another consideration is the context of the link. If you have a page about blue widgets and there is a link via a banner pointing to a site about green widgets then make sure the link is relevant to your page. The danger is that the link may be seen as being off theme and potentially just bought advertising, which could, as we know, open a whole new can of worms.
In short, my feeling is link out with care and if in doubt, nofollow it.