The newish Google custom search feature is quite handy and (from my experience) searches just the forum. Search results can be sorted by date or relevance to your search term. Since it uses Google to query the site, you can use search operators to refine your query based on keywords. Kudos to Istvan and the rest of the people responsible!

As Josh Herr indicates the search form on the homepage restricts the results to this domain and also allows for the Google supported search customization.

We also do have our own search implementation that used to be the primary search but it has been largely supplanted by the custom Google search.

This second search methodology can be reached by clicking the 'similar posts' link under each post. This type of search allows you to filter by content type and is perhaps a little better at restricting results. Basically it restricts each search to individual posts rather than entire pages of content. But in general was found to have less utility than the main search.