Travel headphones need to be...

Koss Porta-Pro or Sporta-Pro

Inexpensive, small, foldable and very good sound for their size. Plus, if they break you can send them back to Koss with a check for $6 and they'll send you a new pair. Lifetime warantee.

Enjoy

second the porta pro--extremely comfortable, fantastic value. the sporta pro, which have a sealed plastic design, just don't sound as good. i also owned the sennheiser px-100 (until consumed by a pitbull)--they were also lightweight and good, though i preferred the porta pro, which had a bit more slam.

Porta Pro didn't work.
Finally finished my headphone amp project based on Texas Instruments 2V amplifier module.. Auditioning earspeakers with my newborn DIY amp I stopped on Ultrasone ICans. The dynamic headroom of these earspeakers allows to use wide power range and they sounded FANTASTIC amplified vs. PX200. Plus completely fatigue free and ultra light weight, completely foldable as well.

I second the Sennheiser PX-100. Great with the iPod, even better with a portable amp.

I agree with all the posts so far - they are good sounding ones, but I stopped listening to headphones, 'cause Doctors are saying they ruin your hearing...
I would recommend trying a small powered speakers to travel with, like audioengine a2 speakers.