In a note that I'm sure the Symfony project is happy to hear, James McGlinn shares the fact that del.icio.us has made the move to use the Symfony framework on their site:
The preview of the new version of hugely popular web 2.0 social bookmarking site del.icio.us has been available (to invited users) for a few weeks now. In the news this morning Yahoo! have confirmed that del.icio.us was rebuilt using the Symfony framework.
The official announcement mentions Yahoo's reasons for the move (including the great documentation and the active community) and their joy at having their framework being put into wide-spread, commercial use.