Assume that the user will have first logged on to all the forums.
The concept is similar to crazedlist.org. A site that summarizes searches from multiple cities and places them on one page, even when there were no items found in some cities. In this case however, the results will be pooled from the search results of various forums.
Since there are many different types of forums out there, the search aggregator should work with the most popular (i.e. phpBB, vBulletin, vBadvanced, Simple Machines Forum, MKPortal, [url removed, login to view], FluxBB ). I will give you 20 forums to develop against.
The code should be readable such that other forums can be added by the user in the future.
The solution can be as simple as required to do the job.. even a properly written google search. (Unfortunately that does not work, google does not consistently return the latest forum postings)
The only test that it mast pass is that it display on 1 page all the search results from the 20 forums given.