[John James Jacoby]

Using deep integration, if you load the WordPress search box into the header of bbPress, you can still search WordPress alone if you use something like

<form id="searchform" method="get" action="<?php echo get_settings('home'); ?>">

This way the search form is using the WordPress root directory as it’s destination rather than trying to redirect to a bbPress file.

As far as a unified search solution, the problem there really then becomes which engine do you use to display the results, WP or BBP? Either way it requires a plug-in on that relative side of the equation. It’s certainly possible, but you’re only making 50% of your audience happy at that point, because the other 50% will want it the other way.