There are not different mixes for CD's and iPods. There the same mix. CD's are generically ripped into a PC and installed into iTunes or another app. Then its transferred to an iPod or another ear-bud player.
There the same mixes, they just got converted to a horrible MP3 bitrate in most cases.
 
Its very possible to mix a song that plays well om ear-buds and huge stereo speakers. Its done all the time and probably being done right now in a studio near you :)
 
CJ

Konrad I belong in the one mix fits all camp. The problem I see with trying to mix to a crappy system is which crappy system to use? All ear buds are not the same so how do you decide which to test and mix for? Ah, you would use the most popular, right? and can you be sure of that? At which given time would that apply? Now in the end you will have a half dozen mixes most of which will sound awful on a good system and the others, well no one will care. But you will have a very bad reputation for producing bad sounding mixes. Of course you could label everything with what it should be played on. "This should be played on crappy systems".  That should go over well.
 
I look at it that what some one chooses to use as their listening device is their problem and should not concern the mixer. His duty is to make the best sounding mix on the best system he can. Those that choose crap will hear crap and those that choose quality will hear quality. To me that is the way it should be.
 
 

I will just look up the definition of good mix somewhere and we won't need to discuss it anymore.

Well said.
 
I'm in the do-it-once camp, too, but if I somehow had a requirement to mix specifically for stock iPod earbuds (maybe Apple wants to preinstall my song in the samples folder) I'd give it a low shelf boost around 80Hz and a gentle broad dip around 4KHz. There's no point trying to boost the low bass.