is itunes using 24 bit yet ?

That's what I do, unless I just can't justify the cost of an entire CD for a single track. There is hope on the horizon as shown by the advent of services such as hdtracks.com. But the record industry is a bunch of retro-minded dinosaurs and still insist on holding onto the old models of distribution, so things are changing only very slowly. n addition, the high (and increasing) popularity and predominance of mobile devices with their low-quality (relatively speaking) headphones as the primary playback method are making it more difficult for making the case that there is a significant market for high-quality downloads.

Regards.

I believe the DAC on iMac and iPhone is still 16bit. Therefore, if you are listening with your "line out" plug, you will be unable to detect any difference playing back 24bit audio. It will be downconverted to 16 bits. To take advantage of 24bit audio, an external DAC is required, which you then connect a pair of headphones to. For instance, M-Audio makes some that can be used via USB.

iTunes supports the 24bit audio files, however the question is, when will Apple being selling 24bit audio? The audio they sell today is 16bit (like CDs).

CD audio and MP3's, both 16-bit formats, are of poor quality when compared to native 24-bit 96kHz audio. This is why I do not buy music from iTunes or listen to music on Spotify and YouTube. Bandcamp is my current place of choice to purchase (24/96) music. My iPhone has a FLAC player app that supports 24/96.