Finally – Google have now implemented a way to request them not to use DMOZ / ODP descriptions in the SERP listings:

<< The way we generate the descriptions (snippets) that appear under a page in the search results is completely automated. The process uses both the content on a page as well as references to it that appear on other sites.

One source we use to generate snippets is the Open Directory Project, or ODP. Some site owners want to be to able to request not using the ODP for generating snippets, and we're happy to let you all know we've added support for this. All you have to do is add a meta tag to your pages. >>

Taken from Inside Google Sitemaps:
http://sitemaps.blogspot.com/2006/07/more-control-over-page-snippets.html

This has always annoyed me because the ODP descriptions are sometimes truly awful and since they're manually edited, getting them changed to something useful can be quite difficult.

For reference, Google's own ODP descriptions are pretty poor (IMHO):

Google.com:
Enables users to search the Web, Usenet, and images. Features include PageRank, caching and translation...

Google.co.uk:
The local version of this pre-eminent search engine, offering UK-specific pages as well as world results.

I don't understand what is the interest to not show the Dmoz description... :-S
Because if it's not the Dmoz description, maybe the snippet that google will made will be worst than the Dmoz description...
no ?

Maybe. But I'd say that at least actual snippets from websites would show some kind of relevancy.

In my experience, some of the DMOZ descriptions the ODP editors have created for my websites have been really uninspiring – once it was just two words – and certainly wouldn't tempt our visitors to click our links in the results pages.