Try the_excerpt_rss() like this:
<meta name="description" content="<?php the_excerpt_rss(); ?>" />
It strips the <p>
tags but doesn’t seem to strip others. For example I had an <abbr>
make it through which kind of messed things up a little.
Of course, you’ll want to put a conditional statement in there to only use the post excerpts for single post pages. Otherwise the first text (usually from the first post) will get posted as the description of your homepage.
Edited for clarity.