Via EarlPearl's post at SEOChat, this appeared on the radar screen today:

It seems Yahoo! is allowing searcher to click the "block" link and remove a result from their searches in the future. This feature is then removable in a similar way by clicking "unblock site" in the results. Speculation about whether this can impact rankings in Yahoo!'s SERPs is only natural, but I would imagine Yahoo! could predict that malicious use would be the most likely result if this turns out to be the case and probably won't use the data to affect rankings, at least initially.

It will certainly be interesting to see if other search engines follow suite, as the convenience for a searcher is quite robust - imagine being able to tag spam sites or results you know you don't want and never having to deal with them re-appearing. I know I'd personally tag about.com - I can't stand their site's design and overwhelming pop-ups and ads.