You don’t need to turn the string into a timestamp in order to create the DateTime object (in fact, its constructor doesn’t even allow you to do this, as you can tell). You can simply feed your date string into the DateTime constructor as-is:
// Assuming $item->pubDate is "Mon, 12 Dec 2011 21:17:52 +0000"
$dt = new DateTime($item->pubDate);
That being said, if you do have a timestamp that you wish to use instead of a string, you can do so using DateTime::setTimestamp():
$timestamp = strtotime('Mon, 12 Dec 2011 21:17:52 +0000');
$dt = new DateTime();
$dt->setTimestamp($timestamp);
Edit (2014-05-07):
I actually wasn’t aware of this at the time, but the DateTime constructor does support creating instances directly from timestamps. According to this documentation, all you need to do is prepend the timestamp with an @ character:
$timestamp = strtotime('Mon, 12 Dec 2011 21:17:52 +0000');
$dt = new DateTime('@' . $timestamp);