I have no idea what is going on here, but I saw a few instances just a few moments ago myself. This was first reported on WMW where a member noticed that Yahoo was changing the titles of his site's listing in the search results. Yahoo was replacing a shortened lowercase different title for its normal title that was Uppercase longer and the accurate original title used on the site.
At first members wondered if Yahoo was just playing hooky and messing with Dmoz titles or even Yahoo Directory titles. This was not the case, as most of the sites mentioned don't have Dmoz or Y! listings.
Join the discussion and outrage at WMW - Yahoo Replacing Site Titles
Update by Barry: I wrote a long post with Danny Sullivan at Search Engine Land on this, hope to have official word from Yahoo! soon.
Update 2 by Barry: This has been fixed now, Yahoo! Fixes Titles in Search Results Sourced From Internal Anchor Text, Thanks Tim!