After asking around for headphone suggestions and receiving some great responses, I decided to go with the Sony MDR-EX71SL earbuds. As I didn't want to order anything online and wait too long, I went with these earbuds, which were easily found at a local Best Buy for around 50 (although they can be found for much cheaper online). They are what you would call in-ear earbuds in that they don't just sit next to your ear but are stably fit inside your ear with silicone rubber ear pieces.
In-ear earbuds have their pros and cons and that depends on what you're going to be using them for. When I first tried the earbuds on after fitting the appropriate ear piece (it comes with small, medium and large sized silicone ear pieces), I was pleasantly surprised at how much noise they blocked out. I immediately thought of the implications of these earbuds on plane flights, Caltrain rides, studying and the like.
Testing
Due to the seal the earbuds make in your ear, bass is impressive and treble is crisp compared to the standard iPod earbuds. I tested the bass in the MDR-EX71s with the standard iPod earbuds with one of the bassier songs in my iTunes library, Timbaland's The Way I Are, with the following EQ settings: dB +10, +4, +2, +3, -1, -1, +2, 0, +3, +6 with the preamp set to +3. It was a world of difference. The iPod earbuds had absolutely no bass in comparison.
To test the treble performance of the MDR-EX71s I picked a song I thought to have some very high frequency parts: DJ Tiesto's Walking On Clouds. I set the iTunes EQ to Electronic and pushed the preamp to +3 dB. I received similar results - the 71s make the iPod earbuds look horrible. Many of you will call me out on comparing the 50 71s to run-of-the-mill iPod earbuds but when you consider that a huge percentage of 71 users are upgrading from iPod earbuds, it's good to compare the two.
Pros
With the proper ear piece, the MDR-EX71s fit very well for me. While running I only had to readjust them a few times and that was because I accidentally pulled them out of my ear. Other than that, I never felt like they were going to fall out.
MDR-EX71 earbuds use a 9mm driver, which is smaller than its non-in-ear counterparts that use drivers around 16mm. However, since there is a seal with in-ear earbuds they are more effective and can get away with the smaller drivers as not much of the sound escapes. In-ear also means that you don't have to crank up your audio device to overcome background noise, thus saving your iPod's battery life (although that is fairly negligible unless you have something like the 1,150 UE-11 Pro quad-driver earbuds which are so sensitive/effective that you are only supposed to use 20% of the volume on an iPod).
Cons
Now let me tell you about the cons of the MDR-EX71 earbuds. My main issue with the 71s is more general to all in-ear earbuds; they are so good at blocking out background noise that using them when running or cycling is dangerous, so much so that the back of the package states:
For traffic safety, do not use while driving or cycling.
I definitely agree. I took the 71s for a run and found myself constantly glancing behind to look for oncoming cars/sketchy people in downtown. With in-ear earbuds you hear more internal sounds: your breath, heartbeat, vibrations on the audio cable, sort of like wearing a stethoscope. I found that somewhat annoying when using the earbuds for sports activities.
Furthermore, the design of the audio cable makes me wonder why Sony even bothered to include an extension cord. It's not like you can use the earbuds without the extension cord as the cable is only a few inches without it. Also, the use of an extension cord means there are two connectors in the middle of the cable, which adds weight. When running I had to hold the connector in my hand or else it would continually try to tug the earbuds out of my ears. I don't think it would be a problem if the audio cable was just one single cable.
For comparison: Standard iPod earbud plug bottom, Sony MDR-EX71SL earbud plug top.
Thoughts
The Sony MDR-EX71SL earbuds offer great value and performance and can be classified as a mid-range product. I would recommend these earbuds for everything but sports activity. They are great for studying in noisy places, blocking out noise on public transportation and iPod listening while walking around campus. I won't be using these for sports activity. While they do stay in your ear, they block out too much noise to be used safely and the ~3.5 foot cord is too cumbersome, requiring that you wrap/hold it.
Perhaps clip-on/behind-the-neck headphones are just better suited for sports? As for non-sports earbud applications, I am awaiting the release of Klipsch's IMAGE in-ear earbuds that are purported to be the world's smallest (and at 350 they aren't cheap either).
The search for the ultimate sports headphones continues. For me all that means is a reasonably short cable, ear pieces that don't fall out yet allow background noise in.