bbmiller - have a read of this. It is rather enlightening regarding burn-in (http://www.innerfidelity.com/content/measurement-and-audibility-headphone-break)
Make sure you read the full article - and especially the conclusions etc on page 4. I particularly love this bit ......
Whew, for a moment I thought that burning in or having used my HD600's over time would cause them to turn into sneakers.