As Brian pointed out, Yahoo Real Estate currently returns broker-uploaded listings by default, but also gives consumers the option to see “MLS Listings” via their agreement with Prudential.

My guess is that Yahoo has (internally) made the decision to end their existing IDX relationship with Prudential at some point down the road and is using the dual option as a way to smooth the transition.

However, it seems like a somewhat confusing option for consumers in the meantime…  Especially, since many agents have “taken the hint” and stopped using the words “MLS” on their websites… opting instead for a consumer-friendly version of “Search for Homes!”