There's a widely used hack for this in blogging software - if you put Code: <!-- MORE --> in your post, the summary is up to that point. Is this the convention you're using? This would also be useful for agregation purposes. In the absence of thos you could always fall back to cutting at the end of the first Code: <p>
<!-- MORE -->
<p>
., ?, !, </p>, </li>, </blockquote>, </div>