Just did a side by side with UM3xs on the new iPhone 5 vs. the 2 year old iPhone 4 (not the 4s). When you first plug into the iPhone 5, there are six clicks that can be heard over a 3-5 second time frame. This happens each time you plug in. There are no clicks on the iPhone 4. There is also a click when you push play for the first time after plugging in which isn't there on the iPhone 4.
The iPhone 5 is not as ambient as the iPhone 4 in an A/B comparison. To my ear, the iPhone 5 rolls off the high frequencies a bit, maybe 3-5 db above 1-2kHz. In comparison, the iPhone 4 is more pleasant and open, with more rosin on the strings, more breath on the flutes, and more brass in the brass. the iPhone 5 is a bit muffled in comparison. I've got the 5 currently running into an open wire at full blast to see if any capacitors "form up" overnight to maybe minimize the low pass effect.
I also compared the iPhone 5 to a 4 week old iPod Classic. The same comparison holds.
So far, I'm disappointed. I got the 64Gb version to hold much more music than the 32Gb iPhone 4. This might be going back in favor of a Samsung S3.
Side note: the screen has a greenish tinge compared to the iPhone4, though that's hardly germane on this post.