I've set the search function to be usable for all site visitors now. The functionality is still a little primitive—it's best to search for a single word at a time. I also don't really like how it looks yet. I'll have to think about where it should go (I suspect it ought to go where the user menu is right now).
The Go button will try to find an article with the exact term you type in. If it can't find one then it will show you the results of a search. Search will only perform a search. If you find the Go button takes you to the wrong place then try again with Search.
Note also that only article titles are searched.