Got it today. I have a bunch of high end portable amps and DACs but the sound of the S3 straight out to my Westone ES5 is pretty amazing. I had the iPhone 4 and now have the iPhone 4S. I also have the Galaxy Nexus which is to be sold. Among all these, I feel the sound of the iPhone 4 to be most accurate, and the iPhone 4S a bit emphasized on the upper bass area - I prefer the sound quality of iPhone 4 over 4S, although the difference is small.
On the Android side, the Galaxy Nexus produces the best mid range and has the best synergy with my ES5 in my experience, but the Galaxy S3 is, IMO, the best sounding of all the recent phones I have had, as far as my ES5 is concerned - very natural albeit a bit less accurate - more "analog" sound, with an obviously wider soundstage than my iPhones. The dynamics on the Galaxy S3 is also better than the 4S IMO. However, compared with the Galaxy Nexus, the vocal from the Galaxy S3 is a bit less intimate. Anyway, when I listen back to my iPhone 4S with my ES5, its sound is a bit too accurate for me to the point that it is a bit "digital".
Of course, when it comes to use with external DACs and amps, nothing beats the 4/4S, simply because it has dedicated portable DACs on the market, namely the CLAS and the Fostex HP-P1, whereas there is no dedicated portable DACs (or any DACs) for any Android so far. But for pure IEM use straight out from a phone, I find the Nexus -> ES5 combo and the Galaxy S3 -> ES5 more satisfying than the iPhones, but, your mileage may vary.
With the Wolfson DAC on the S3, I am naturally wondering whether there will be Voodoo mods like they did on S1. I never owned an S1 but heard that the sound quality from Voodoo on S1 is the best one can ever have on a phone.