Heres a googled up forum thread that mimics what I think about the beats line of headphones:
http://forums.macrumors.com/showthread.php?t=699834
My brother (a hip-hop kid) loved the shit out of them in the BB kiosk. When I had him listen to the same thing on an open air Sennheiser HD555 (plus decent DAC and homebuilt amp) he was blown away.
I agree with the earlier poster that I couldn't get them off my ears fast enough (the beats). They are not studio quality nor are they worth the money. WAY too much bass, but definitely comfortable!
This. Not a fan of the whole beats thing...
OP: How portable do you need to be with your headphones? One thing an iphone has is the LOD (line out dock) which will allow you to easily use any headphone amp to drive your cans. I'm pretty anti-apple, but this is one thing that I wish other phones/mp3 players did.
Edit: To give you an example, I use Sennheiser HD650's. Well regarded as a solid choice around the 200-300 dollar price point (definitely not the only choice). I typically use mine when at work and at home. They're open ear, which means you'll be able to hear things around you and people around you will be able to hear what you're listening to. Definitely not for an airplane ride or something like that. Are you going to use your headphones in an open or noisy environment? If so, you'll probably get more use out of closed ear headphones. If you're doing a ton of travelling and such, maybe even a pair of IEM's (in ear monitors, like the cheapo earbuds that come with the iphone/ipod, but a ton better and actually isolates sound so it will block a ton of noise as well).
Mine need to be amped, and since I mainly use it in 2 different spots, I use the Fiio e7/e9 combo. The e7 is a DAC/small amp, which allows me to bypass the internal (crappy) soundcard of my work laptop (and my home desktop for that matter). The e9 is a big amp (that the e7 plugs into) to make the HD650's sound even better when I'm at work (I leave the e9 at work since its kinda clunky, then use the e7 by itself when I'm at home).
I use this because almost all of my music is coming out of my computer (hence the desire for a DAC). You, on the other hand, if more mobile, will probably want to go with a dedicated portable amp that works with well with the iphone. Someone else might be able to chime in, does the iphone have an EQ?
Either way, a small amp with some good headphones sounds like a solid choice for you. Didn't mean to ramble about what I had, mainly just saying it for education purposes.