It appears that within the past 2-3 days the popular social book-marking site del.icio.us has started blocking the major search engine spiders from crawling their site. This isn't a simple robots.txt exclusion, but rather a 404 response that is now being served based on the requesting User-Agent.
This raises some questions as to the intentions of del.icio.us, and perhaps Yahoo! With Yahoo! recently integrating del.icio.us bookmarks into its search results this could an attempt to enhance the effectiveness of that new feature by preventing competing search engines from indexing content from del.icio.us. [More]