Know what REALLY annoys me? People listening to music on freaking CELLPHONE SPEAKERS! That's even worse than earbuds, because at least you get a stereo image (and probably some low-end) through earbuds! And most of the culprits are kids listening to hip-hop (read: bass-heavy) music...and they're not even hearing the bass! 

But when you think about it, most of these folks are listening to music they already like and thus have already heard multiple times. They have most likely heard that Lil' Wayne tune in a club or in a car with the boomin' bass. So when they're listening to it on their tinny little Galaxy II cellphone speakers, they're really just reminding themselves of the song and not really *listening* to it.

Well, I am pretty sure I am getting better sound from my better-than-average-but-not-over-the-top occlusive ear buds than I was getting on an 8 track driving down the highway. Most people who listen to pop music have always been pretty tolerant of imperfection in the playback system. These days they usually compensate for poor transducers by upping the output until their eyes bleed. That solution leads to the second reason they don't demand audiophile quality mixes...deafness. Perhaps you should amp up the highs in your mixes to stimulate the few remaining hair cells in their hammered cochleas.

There are two sides to this argument. Yes most people these days including hardcore music fans don't listen on dedicated stereo systems anymore. Its all mobile and increasingly on phones themselves. This is more a reflection of peoples lifestyle and the easy accessibility of music on the go which we never had before. 

The plus side is that earbuds have gotten a lot better these days. Anyone remotely interested in music typically gets a nicer set of earbuds or headphones that sound pretty good. The earbuds I got recently rival the sound of my ATH-M50's so a decent mix sounds pretty good even through my phone. Arguably better than what it would sound like on a subpar stereo system.

The thing that bothers me most is the fact that a lot of music just sounds more natural through speakers and not through phones and we don't have any control over that. 

So the general advice is to definitely spend a lot of time on making your mix sound great on headphones since most of your target audience will be using that.