Most headphones get better with age, so to speak. Some say the burn-in on most cans is roughly 100 hours. I noticed significant improvement on the soundstage and bass response of my Sennheiser PX200s after about 70-80 hours of use.

My E4cs have had about 50 hours or so and not really much change. For the first new nights, I left them plugged in to my hi-fi with music going through at a slightly higher-than-comfortable volume. Some say spoken word is better, or even white noise, but who's to know?

Edited to add: also, don't forget: using a pair of Shures or Etys with an Ipod or most other mp3 players is somewhat overkill, I would say. Regardless of the bitrate of the file you're playing (even lossless), the circuitry of an Ipod is, in my opinion, simply too sub-standard to get the best out of any high-end headphone/canalphone. I had an Ipod Mini for 4 hours once. Gave it to my wife because the sound, compared to my Minidisc, was simply awful.