The first thing I see is that you are trying to float divs inside a P tag, which is never that great of an idea:

<p class="meta">
etc..

I would first turn meta into a div. Then instead of using a single div to float the date left, use one for the left data, and one for the right, clearing after them. Something like this:

<div class="meta">

<div style="width:49%; float: left;">
  (post date code)
</div>

<div style="width: 49%; float: right;">
  (posted by, leave a comment, etc..)
</div>

<div style="clear:both;"><!-- --></div>

(categories and trackback links here)

</div> <!-- end of meta -->