The iPhone 4 and iPad 1/2 were used as the PRIMARY tuning sources/amps during development although we used many others of course, especially pro DJ gear from Allen & Heath, Pioneer and NI. IMO the iPhone 4/iPad has quite amazing "vivid" midrange and flat response curves as source amps and DACs, way beyond the previous gen iPod and iPhones. The iPhone 4/iPad trumps even the Macbook Pro I had during development and that I use for music production, I always convert the song to AIFF and send it to my iPad or iPhone for critical listening. Firmware/Software versions of OS and the player software can change sound drastically, we learned this also with our V-MODA MUSIQUE app for iOS + Android.
Big kudos to Apple, I can't wait to see what the iPhone 5 has up its sleeves... I'm trying to find the best Android & Windows phone and software combo too, but so far the sound just seems "thinner" on the Androids amps I've tried.
-v