Two options, best first: he Sennheiser HD 448 are probably more suitable for portables, having a more mobile-friendly 1/8" plug standard, good, balanced overall sound with a very respectable 16-24kHz response, sealed over-ear design, and a mobile-friendly 32Ω impedance. You can also get 'em just shy of $100 or less, which is great for Senns. The bass is a little on the soft side and it loses a little shine above 10k, but that only really matters if you're a freak about acoustic detail. Then there are the bigger, meaner Sennheiser HD 555s. 50Ω impedance, so not ideal for mobile, but you probably never listen at full volume on regular cans anyway, so you can bump the volume up to cover the extra impedance to get a great overall sound with plenty of volume still to satisfy. I use these as my studio monitors when I'm composing, and they're great. Unfortunately for mobile users, they terminate in a 1/4" plug and you have to use an adaptor (included) to get it down to 1/8", so you'll have a funky long plug coming out of your iPhone/iPod, which may not be ideal for you. You'll also pay a few sawbucks more for these.
