I use xml escaped html in my description field (if I put anything at all).
Code:
<p>This is the first paragraph.</p><p>And this is the second paragraph.</p>
I have no idea if there are readers/apps that can't deal with it, but I've seen it in a lot of commercial ebooks.
I've also found that many readers like setting the Subject and Description metadata tags themselves. So most of the time, I don't include anything at all for those fields. Those that care about them are going to make them their own anyway, and those that
don't care about them ... well ... they don't care about them.