I largely agree with the answers posted above. I would add, however, if you're going to be doing any processing of the audio (versus simply sending the original bit stream to the DAC for listening), then 24 bit can have an audible advantage in certain circumstances.
For example if you have say two versions of a classical recording with a wide dynamic range--one in 16 bit and the other in 24 bit. And say one of the tracks is very soft and you wanted to normalize or "volume level" just that one soft track to put it on your iPod. That might require 15+ dB of gain be applied. In such an instance, the 24 bit recording may yield an audibly better result as it may have a lower noise floor.
But, with many recordings, the actual noise floor of the recording is usually the limiting factor. This includes the microphone preamps, the A/D converters, any mixers used, signal processing software or equipment, etc. And there's also the inherent noise in the playback signal path and ambient environment. Headphones that offer significant isolation are especially relentless at revealing noise as they can lower the ambient background noise by 20+ dB.
Increasingly, music is being recorded with a very short and limited analog signal path. The microphones (and/or musical instruments) are often connected to high quality A/D converters and each is recorded digitally to it's own track--typically in 24/96 format. All the mixing and signal processing is then done using software like Sound Forge, Pro Tools, Wavelab, etc. And those tools typically use 32 or 64 bit internal processing to avoid degradation, rounding errors, etc.
So it's certainly possible to produce recordings with a really low noise floor. And, if you crank in enough gain at certain points along the way, 16 bits might not be enough to prevent audible noise. But will 99% of Apple's 24 bit files have an audible advantage? I doubt it.
The interesting thing, to me, will be if Apple uses DRM and/or some proprietary Apple-only format, for their 24 bit tracks. If they do, it will be more difficult to do a blind comparison than simply using something like Foobar and ABX. Otherwise, with an open format, it would be trivial to verify if they have an audible advantage. And, so far, most blind listening tests of 16 vs 24 bit have found no advantage to 24 bit.
Edited by nwavguy - 3/8/11 at 10:52am