Jacob Dehart has created the perfect tumblr search engine. It does not require pages to be indexed by google or yahoo and is based on the Tumblr API meaning if its on your tumble-log, it will be found on the search. The best part is you do not have to re-load another page or click a button to view the search results. You can see the search engine live on my tumble-log Ziked. So heres a tutorial on how to install such a search engine for your tumblr page.
Updated: It seems that NING does not allow users to post html code (even with the "XMP" tag). SO if you are interested the tutorial can be found here