Ok, the way I see it is true XHTML is what ever is going to validate under the Strictest Rules of XHTML.
So then i made up a little markup that can be ran at
http://validator.w3.org/
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w
+3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="en-US" xml:lang="en-US" xmlns="http://www.w3.org/1999/xht
+ml">
<head>
<title>HTML BR TAG</title>
</head>
<body>
<p>This is Good<br /></p>
<p>This is wrong<br><br/></p>
</body>
</html>
Scroll down till you see "Validate by direct input" paste the markup above in it, select "Use Doctype XHTML 1.0 Strict", you can also check "Show Source" and "Clean up Markup with HTML Tidy" HTML tidy will show the right markup to use.