I've created a custom search engine for all things DITA using the Google Co-op search. The search engine homepage is: http://www.google.com/coop/cse?cx=015401811188033694464%3Ahu_dvbft4pu

I've also created a DITA Documentation search (lang spec, toolkit, other official docs) here:
http://www.google.com/coop/cse?cx=015401811188033694464%3Acm02go-4hv8

I've also created a search plugin for Firefox. Details on the plugin are available here:
http://shudson310.blogspot.com/2006/11/dita-search-and-firefox-plugin.html

--Scott