There are regular independent updates on each search engine’s share of use, such as June 2008 Search Engine Market Share | Nielsen // NetRatings in E-Marketing Performance, but you need to be careful with these statistics and read the small print. Firstly, they’re usually from the US, and things are different here in Europe. Secondly, they cover all web users, and search engine use may be (and indeed, is) different amongst professional users.
In fact, Google is even more prevalent than its competitors in Europe, and the lead is extended even further in trade and techinical markets. So if your site analytics are showing that 90% of search engine traffic comes from Google, don’t be too concerned that you’re underperforming in Yahoo! and MSN/Live Search. It’s probably about right.