we’ve just deployed instant search to bugzilla.mozilla.org:
it uses the possible duplicates function that you see when filing a new bug to display results as you update the search term. while it’s much quicker than other searching mechanisms, it’s also very limited – only the bug’s summaries are searched.
one item of note is it uses the related products facility from the guided bug entry form to automatically search related products; eg. if you select firefox as the product, the core and toolkit products will also be searched.