My favorite over-the-ear headphones for travel are Sennheiser PX100s because:
-- they are relatively inexpensive (though they are now twice as expensive as when I bought them)
-- they collapse into the included carrier, which is quite durable
-- they are easy to power, so sound good driven directly out of the iPod
The PX100 is an open headphone, so it won't block outside noise at all. The PX200 is sealed, so it will block out some noise, but it doesn't sound as...