Google has always been the default search engine in Ubuntu's Firefox, but now it's changing to Yahoo beginning with Lucid Lynx. This doesn't have to do with Google's work on Chromium OS or the Chrome/Chromium web-browser, but rather a business arrangement between Yahoo and Canonical. Canonical is changing the default search path over to Yahoo as the two companies have negotiated a revenue sharing deal off the advertisements when using Yahoo search on Ubuntu. Users can still switch Firefox to using Google search with a couple clicks, but Canonical is hoping this change will yield them some additional revenue.
This was made public today on the ubuntu-devel list.