Yeah, it happens to everybody, or at least should. Its how your brain works. Did you know that if you have a white noise track playing in the background (that static sound that plays on your tv if its tuned to a channel where nothing is coming in), and you aren't actively paying attention to it, that your brain will eventually just "tune it out" and you have to try to pay attention to it to hear it again. If the brain can tune that out, it can tune out the defects in iPod earbuds.
However, what the brain can't do is make bad headphones pick up details that they just aren't able to pick up. Your brain can "tune out" some noise, but in the process you're going to start losing some detail. That's why the "sound" is okay, your brain justs adjusts, but you also sacrifice a lot of detail. Where with good headphones, there isn't as much adjusting for your brain to do, so you hear more detail.
That's way over-simplified, but it gives you a basic idea. THis mainly applies to phones that bring a lot of noise into the equation or have weird spikes. It also tends to work better on stuff you are familiar with, or at least sounds familiar. That is, your brain kind of knows what its supposed to sound like.
