Can iTunes now play HD music files in high definition?
iTunes uses the same technologies as QuickTime for playback. 

According to this document circa 2005, QuickTime is capable of playing 24-bit , 192kHz audio files. I think it's safe to say that iTunes is fully capable of playing these files.

However, you're probably limited by the hardware in your Mac. If you open the "Audio MIDI Setup" app, you'll probably find that your Mac is limited to audio output at 96kHz.



So, if you want to listen to these files in full 192kHz, you'll need to use a dedicated external USB DAC, and bypass your Mac's internal audio hardware. Please correct me if I'm wrong.

Further reading: http://www.head-fi.org/t/671361/how-to-get-24-bit-192khz-files-from-my-macbook-pro-to-play-through-the-dac-of-the-fiio-e17