You could include the first few characters of the post in the title, by simply adding some Smarty code to your entries.tpl, like this;
<h4 class="serendipity_title">
<a href="{$entry.link}">
{$entry.title|@default:$entry.body|
truncate:200:" ..."}</a>
</h4>
The reason the truncate has been set at 200 characters is because this allows even very long titles to be used, however it also means that any entry that does not have a title, will now display the first 200 characters of the entry body.
An alternative, is to use the entry id instead of the first few characters, and this is the approach I'm generally following with all my recent templates. Its an elegant solution, doesn't ruin tight designs, and fits with the fix Garvin applied to the recent entries plugin which wouldn't display a link to the entry if the title was missing.
<h4 class="serendipity_title">
<a href="{$entry.link}">
{$entry.title|@default:$entry.id}</a>
</h4>


