yup, jbarker mentioned it.
i'm actually using a flat response headphones right now to listen to stuff after midnight to prevent disturbing my sleeping dad.. haha it's a audio technica ATH-M40fs
it's not bass enhanced so in a way, it's not the best sounding headphones around for music listening. but it's because of this property, it doesn't lie, so badly EQ'ed sounds will sound bad and well EQ'ed sounds will sound acceptable. i use this mainly to tweak my tones from my multi effects unit and also to EQ the audio from my video clips i take with my digital camera.
conventional theory is that things that sound acceptable on a flat response headphones sound good on audio enhancing sound systems. however, things that sound acceptable on an audio enhancing system may not sound so good on every sound system.