Colin Cochrane noticed that del.icio.us has blocked search engine spiders. He believes that it's not a simple robots.txt exclusion; instead, del.icio.us is serving 404 errors based on the User-Agent. Barry Welford confirmed this by changing the User-Agent himself.
How did he come across this? He was using a Firefox del.icio.us addon that and couldn't locate a page he had referenced before. It was only when he did the search directly on del.icio.us that he found it.
I honestly hope that that is not the case.
But EGOL says something else. It is possible that many people are gaming their way onto the del.icio.us front page (heck, I've seen some pretty bad-quality sites there myself) and this is the way to not pass juice to them. It's not the most ideal solution, and it's a mistake to do this without being forthright.
Most people believe this is just a bad mistake made on Yahoo's part, which doesn't help since Yahoo has been having a difficult time lately, and this doesn't help matters at all.
Forum discussion continues at Cre8asite Forums.