From our FAQ section
FAQ #9: Do I need a Google & a Yahoo Sitemap file? Is a Yahoo Sitemap file important?
The Google Sitemap file based on the XML protocol was so extremely efficient for Google that Yahoo jumped on the bandwagon and collaborated with Google to adopt the same technology. It is the first time to our knowledge that the 2 main competitors have cooperated in a long time on such an important technology.
It is obvious that you should get a Google Sitemap file as a priority, but a Yahoo Sitemap file is also very important. Amongst major search engines, top competitors constantly spy on each other’s database. Yahoo is the only real competitor to Google. Although much smaller in terms of overall market share, we strongly advise you to not only sign up for a Google Sitemap file, but also a Yahoo Sitemap file. Once again, it is a one-time fee.