The very first Tip at the top of pg 75 of Liz Castro's HTML XHTML & CSS (6th ed.)
Text and inline elements should not be places directly between the opening and closing blockquote tags. Instead, enclose the text and inline elements in a block-level tag--like p, for example--within the blockquote tags.
I didn't know that either. Which is why I got the book... So maybe putting <p> and </p> around it will fix things. Perhaps if I knew how to read XHTML schemas, it would tell me that <br /> could only appear inside a block-level tag - but I'm too lazy to parse schemas.