Ok, done :) There's some good news & some bad news.
Firstly, it's not your fault. The bug that is causing the problem on your site is in the "Pages" widget of the Sapphire theme. See below:
<div id="pages" class="widget widget_sapphire_pages"><br />
<li class="page_item"><a href="http://planetpooks.wordpress.com">Home</a></li><br />
<li class="page_item"><a href="http://planetpooks.wordpress.com/about/" title="About">About</a></li><br />
*** Closing pages DIV tag missing here ***<br />
<div id="archives" class="widget widget_archives">
I've replicated this on my own blog, by changing to the Sapphire theme & adding the "Posts" widget - other's should be able to replicate also.
So that's the good news. The bad news is that you'll have to wait for this to be fixed (I've submitted a Feedback bug report).
In the meantime, if you *really* want to use that theme, just remove the Pages widget.