You can append post_type to the end of your search string like this:
http://yourdomain.com/?s=search+string&post_type=page
This works like a charm:
<form method="get" action="<?php echo home_url();?>">
<input type="text" name="s" />
<label>Search Pages</label>
<input type="radio" name="post_type" value="page" />
<label>Search Posts</label>
<input type="radio" name="post_type" value="post" />
<button type="submit">Submit</button>
</form>
You could also do something like this within the form tag (sans the radio buttons) for contextual searching:
<?php if(is_page()):?>
<input type="hidden" name="post_type" value="page" />
<?php endif;?>