Why, it's the same as MSN's... "Google." But, Yahoo! is fighting back by placing a Yahoo! search box at the top of the SERPs on a Google search:

I really do feel bad for Yahoo! on this issue. I can't imagine how frustrating it must be for Yahoo!'s engineers to endure search logs that are filled with "Google", "Google search", "search Google" and the occassional "myspace" or "Jessica Simpson"... I can see how that might get old very fast.

I'd love to know how effective that "onebox" result is... Can anyone from Yahoo! comment?