There is no quick & easy way to search for closed bug reports
Searching for closed bug reports is really painful in Launchpad. You have to go through the "Advanced search" form and select all the bug statuses available.
Side note: although this particular bug is about the inability of searching for closed bug reports, what I'd like to have is a way to look for both open and closed tasks. I think the best way to achieve this is adding a link "Search also for closed bug" to the results page.