bold in quotes is my edit
I thought about what "nofollow" means/implies for a while and I don't think that it means "I don't vouch for this link" but rather " I don't vouch for the content that this links points to". It might be splitting hairs but there is distinction especially with the sites with user contributed content, which plays into putting nofollow on your own links.
Take for example G's own video page video dot google dot com - if you look at all the links to popular categories (top 100, comedy, etc) you will see that they have nofollow on them (even though they point to sites on G's own (sub)domain). However if you follow any of the *category* links you will see that once there links to individual videos do not have nofollow on them. That is in contrast to main video page where selection of links to individual videos have nofollow on them.