Get the Sennheiser PX100. Couple sites claiming that it's actually better than the PX200. Gonna purchase them myself. At $40, it seems like a great buy. Anyhow, be sure to read these:
Don't buy the Sennheiser PX100s. I have a pair and although they're quite nice for the price, they won't block out external noise. And those around you will hear what you're hearing.
For your price range and your need for a comfortable, isolating headphone, I'd stick with the Sony MDR-V6 which can be had for about 70 bucks. If you can make it to 100 bucks, my choice would be the Sennheiser HD 280s.
Since you haven't bought your Iriver yet, you might want to also look at a Rio Karma before you purchase. You might be pleasantly surprised.
I guess I want the best noise-cancelling phones in the size of the PX-100-250 line. I was seriously looking at the V6's but I just can't see myself putting those cans on my head. hehe
Kyle, guess you're going to need to spend some money for a decent pair of noise-cancelling headphones. Or the Sony V6 seems to be a good choice, as everyone seems to recommend them. It seems like all noise-cancelling headphones under $100 sound like crap.
Picked up the IRiver today. I brought in an ad from another store and got them to mark it down to $299. Plus I had $20 in Best Buy rewards money and a 10% coupon. So I got a pretty good deal. Now I think I'm going for the Sony V-6's
I had the Shure E2C's and I thought they were great. Although I have never used them on a plane, in a normal setting (in NYC) they cancel out most noise pretty well. They are very comfy and sound very exciting (or musical if you will). Great for portable use but not as much so at home and that is the only reason I sold them. For about $75 I think they are a hell of a deal.