On monobook.php, line 101
replace <h1 class="firstHeading"><?php $this->data['displaytitle']!=""?$this->html('title'):$this->text('title') ?></h1>
by <h1 class="firstHeading"><?php str_replace(" ", "_", $this->data['displaytitle']!=""?$this->html('title'):$this->text('title')) ?></h1>