This is a toughie, and it's a bit dangerous, too: it can yield an invalid document. Consider this starting point:
By replacing the BR with /DIV DIV, you'll end up with this:
<p>Here is a line of text<br />
Here is the second line.</p>
There's a lot of parent-element validation involved in this one.
<p>Here is a line of text</div>
<div>Here is the second line.</p>
BR isn't going to be deprecated any time soon, since the W3's HTML WG lost its backbone and caved with XHTML 2. It was to be replaced the <l> (L) element, for Line, but they've received a bunch of comments declaring that they're not the same thing, and that usually results in the inclusion of both. At least with the HTMLWG.
Whoa...sorry. Tangent over!