I've run into this problem in tables. It is not valid XHTML to include p tags within td tags but if I try to add a blank line between paragraphs in a table cell WordPress interprets that as described in the article at http://wordpress.org/support/topic/150919?replies=16 and replaces the two br tags with a p tag. I tried adding a style="height:4em" to the br tag to no avail. Adding style="margin-bottom:2em" and style="padding-bottom:2em" didn't work either.
So how do you get a blank line in a table without using two br tags?
Note that I'm using Windows Live Writer and it has its own HTML formatter which produces semantically correct HTML but I don't have any control over that formatting (just in case someone says to remove the space between the br and the slash - can't do it with Live Writer).
Thanks,
David