Bernard Mangold has the announcement on the Yahoo Search blog about Yahoo MindSet, a prototype that reorders search results "according to whether they are more commercial or more informational" as you move a little slider around.
It's pretty similar to but more limited than Google Labs Personalized Search, which allows searchers to check boxes for their high level subject interests and then gives you a slider to reorder search results.
MSN Search also has some similar sliders (though they don't reorder dynamically) on their main search site inside their Search Builder advanced interface.
I'm surprised to see this focus on sliders. They aren't particularly useful. They fail the grandma test. Most novice users will not use or understand the slider; they just want the top result to be useful. It's not even that useful to power users since sliders fail to provide the level of granularity they need.
More knobs, more buttons, more complexity. With less than 1% of users even bothering with the existing advanced search options on search engines, are more controls really what people need? Most searchers just want the right thing to happen. They want it to just work.
See also my earlier posts, "Personalized search at PC Forum" and "Peeking at the future of MSN Search".