I just got $300 to spend at the Apple store. I think I want headphones, and they sell what have been my Holy Grail, the B&W P5. But I'm going to open this up to anything the Apple store sells in the same price range. Audio source is an iPhone 4, and I have no interest in keeping an amp clipped to my belt when I'm on the go. These have to sound good straight out of a source; I don't have the luxury of owning anything resembling a good stereo - my best stereo component is a vintage SonicImpact T-Amp and THX-certified PC speakers (but their internal headphone amp is noisy!) I'm leaning toward mostly semi-mobile use; they should be comfortable enough to wear for hours, and both small and durable enough to throw in a backpack with a laptop (cases help durability, but hinder portability!). They may be used connected to a desktop PC with or without a USB sound card.
Another relevant point - I prefer an L-shaped plug to a straight one; the Beats low-profile plug is about ideal. I may eventually buy a Beats cable to use on whatever headphone I end up selecting, if that's feasible.
Here is my shortlist to prime the discussion: