As others have already reported, InfoSpace's Dogpile metasearch engine added MSN Search to its layered results. Here's a visual tool where users can compare the uniqueness of search results across the major engines.
Dogpile, with its ability to segregate results by engine, is right now a power user's tool. The challenge is to build traction among ordinary consumers, who don't necessarily understand the concept of "metasearch" — though the public does get the concept of travel metasearch (e.g. Orbitz) and comparison shopping engines.
Related: Chris Sherman's write-up of the InfoSpace-sponsored study of divergence among search engine results on Search Engine Watch