50% of the search traffic is captured by Google with about 25% going to Yahoo and about 20% to MSN. A rather small insignificant % goes to several smaller search engines.
Generally speaking, if you optimize your site to rank well long term for Google, you’ll do well in Yahoo and MSN as well. Yahoo and MSN results tend to be easier to break into compared to Google. This will change eventually as search technology continues to improve and get more sophisticated.
The key is to balance your traffic and not rely entirely on any one major search engine for sales revenue and profits. Technology changes quickly and while Google is still the market leader, there may be another stronger competitor who enters the market and captures the search market share.
Following accepted industry-practices will keep you out of trouble and in many cases help to keep your site in the top results for a long time.