I dont believe iTunes alters the audio either but they do have a feature called 'Sound Check' that is built into their playback software. It was mentioned in the SOS article I think. Apparently they were not prepared to give up what the value was but Bob Katz seems to think they are normalising all material to a -16.5 LUFS standard.
 
So this is roughly 6 dB higher than -23 LUFS. So iTunes analyses the overall level and makes an adjustment. If a track is at -23 LUFS then it is adding 6 db or so. But tracks that are heavily mastered to be very loud end up getting huge amounts of attenuation. What ends up happening now is those tracks end up sounding a little quieter than others that are not slammed. They also end up sounding 'small'. Great news I reckon.
 
Hopefully it means that mastering engineers once they realise this will have to stop mastering so loud once and for all. Apparently 'Sound Check' does a rather good job with lots of material and makes everything playback at a much more even volume etc. It is meant to not change the dynamics within the music just raise or lower the overall level to meet its own standard.
 
 
 

Things like Soundcheck and ReplayGain can be thought of as just a flag added to the file to adjust the volume up or down for you automatically based on analyzing the perceived loudness.
 
For something like a multi-movement piece all that needs to be done is to analyze the whole piece or album and set the flags appropriately for each track based on the whole piece or album.
 
http://en.wikipedia.org/wiki/ReplayGain