It appears that Yahoo! is now bolding the domain name in the search ads on the page.
For example, a search on ipods brings up dozens of ads, at the top and right side of the page. If you look at those ads, the domain names, under the ad descriptions, are all bold.
Here is a screen shot of the top ads and some of the right side ads, all showing bold domain names.
Is this a bug or new feature in Yahoo! Search?
Forum discussion at WebmasterWorld.