I finally figured out how to make a useful 404 page for my sitefinity sites. Before, since it was using the asp.net error redirecting, it was returning a status of 200 instead of 404. This was simple enough to fix (changing the Response.StatusCode) but in addition to that I can now use the Sitefinity search API to display a list of possible related pages for the users!
Read more on my blog entry:
sorry about any formatting errors; I just updated my blog and am sorting through the css and stuff. any comments or criticisms are appreciated!