The question about ODP using the Nofollow tag has been raised recently in various places.
For example in this SEO forum thread: Should DMOZ switch to nofollow

What is nofollow? In normal circumstances a link from one site to another is used by search engines, such as Google, MSN and Yahoo as a “vote” for the validity of the other site. Most people agree that in the case of Google, it contributes to page rank, and helps to make the second site more visible in searches. It has been suggested that search engines will now pay attention to the nofollow tag, when present, and ignore that as a meaningfull link. Thus a webmaster can downgrade the importance of a link by adding that tag.

Those who are upset at the fact that DMOZ does not list their sites in a timely fashion (whatever that may mean), and do not like that Google pays attention to DMOZ, want the ODP to add the nofollow tag to all it’s listings, so that those sites that are listed do not gain advantage over sites that are not listed, and to cause the DMOZ listing to be less important to Google.

What’s wrong with this concept?

• If Google is doing the wrong thing, then you should complain to Google, not get DMOZ to try and manipulate Google
• Adding nofollow only affects the DMOZ directory and very few referrals come directly from it
• Many more referrals come from third party users of ODP data, and those users get the data from the RDF dump, which is a formatted XML file. This file does not contain any HTML code, so the nofollow tag would never be in there.
• Why should the ODP add the tag at all. Since sites are put in the ODP because editors think they have relevance, putting nofollow is a contradiction - saying in effect - the site has no validity.

Other links related on nofollow

googleblog - Preventing comment spam

Six Apart - Support for nofollow

SEObook

SearchEngineWatch

Wikimedia - a discussion of whether they should use nofollow

padawan.info