I am 99.999% sure this is a new user interface for Yahoo Search. In the past, Yahoo never ever indented search results. In fact, in the past I thought they did do indenting and then stopped, but Yahoo told me that they have "never grouped results."
Many search results, including a search for search engine roundtable return grouped results now. Here is a picture:
This is 99.999% new behavior from Yahoo Search.
Forum discussion at WebmasterWorld.