OK, let me just jump in to correct some misconceptions since I'm an EE student so I do have some knowledge of the subject.
A) 44.1 KHz is a sampling rate, and by he Nyquist sampling theorem, means that you can reproduce sounds up to 22.05 KHz. Without going into great detail, basically when you want to record you filter any higher frequencies than Fsampling/2 before you do the sampling or you will get aliasing artifacts.
B) I'm sure there are sounds that have harmonics at higher frequencies than 22 KHz, but its debatable as to whether anyone can perceive them, regardless of duration. Cymbal crashes and other abrupt sounds contain a lot of high frequency components, again whether you can perceive anything above 22 KHz is very debatable.
C)Any digital storage of analog signals is susceptible to quantization error. I'm not sure how the specific CD-Audio spec goes about quantization, but I vaguely remember reading somewhere that the scheme allowed for more error at high frequencies since the difference is less perceptible to humans. (google "quantization error" if you want a definition of that).
D)D/A conversion is usually far from perfect. Depending on the quality of the DAC in your system you will get varying quality from an identical digital source. Obviously, most consumer-level stuff uses schemes that are inexpensive and sacrifice some level of quality.
E)earphone specs above 20 KHz are pretty inconsequential, you may as well ignore them. At 20 Hz, you can't really hear sound as much as you feel it, as the saying goes "there's no replacement for displacement". Even the best headphones cannot rival the "feel" of listening to a track on speakers with a subwoofer that pumps accurate, gut-wrenching low frequencies. With that said, look for headphones that retain a flat response across as much of the audible spectrum. With that said, it could very well be that the Sennheisers are better cans than the stock Apple ones.
F) Don't bother with manufacturer THD specs unless you can get the level at which these measurements are taken. These numbers usually can't be compared across brands most of the time because nobody publishes the level at which the measurements are taken.
G) AAC is technically superior to MP3. Period. AAC at 128 kbps sounds better than even LAME-encoded MP3; you can definitely hear a difference at 128 kbps with decent speakers or cans. At 192kbps or above, things become murkier, and in general it becomes difficult to tell codecs apart at that bitrate and above. Unless you're listening to the tracks in a very quiet room on high quality speakers most codecs are essentially transparent above 200 kbps or above on most pieces. Some people claim they can hear imperfections in encoded audio on certain pieces even at 320 kbps; to be perfectly honest, pretty much everything is transparent for me above 192 kbps with my setup.
H) Remember that there are diminishing returns in performance/cost as you get to more expensive equipment. See if you can try out headphones before you to see if they really make a worthwhile difference to you.