As expected, Jimmy Wales, founder of the Wikipedia, has launched his search engine, named Wikiseek.com. (This is not a search engine by Wikipedia, but it does use Wikipedia's content - sorry, still catching up). All I have seen are negative reviews about the search engines, you can read Danny's huge analysis at Search Engine Land, his conclusion:
In the end, if you want to search Wikipedia, just go to Wikipedia and search there rather than Wikiseek. That seems the better experience. Or search at Google -- it tends to bring up Wikipedia pages all the time for relevant queries.
The overall consensus is that it is currently a poor search experience because of the restrictions of only showing results within the wikiepdia community.
I also did notice someone at WebmasterWorld noted that http://wikiseek.com/ returns a 404, they need to 301 that to the www version.